Aluminum alloy plate having excellent bake hardening responses

ABSTRACT

One of the purposes of the present invention is to provide a 6000-series aluminum alloy plate exhibiting bake hardening (BH) properties and molding properties after aging at room temperature for a long period. In one embodiment of the present invention, in a Sn-containing 6000-series aluminum alloy plate, specific clusters that greatly contribute to the development of the BH properties, which are determined by means of a three-dimensional atom probe field ion microscope, are contained at a predetermined density or more, and the sizes of the atom clusters that meet the aforementioned requirement are uniformed to adjust the average radius of an equivalent circle diameter of each of the clusters to a value falling within a specified range and reduce the standard deviation of the radius of the equivalent circle diameter, whereby the BH properties after aging at room temperature for a long period can be improved.

TECHNICAL FIELD

The present invention relates to an Al—Mg—Si alloy sheet. The aluminum alloy sheet referred to in the present invention means an aluminum alloy sheet that is a rolled sheet such as a hot rolled sheet or a cold rolled sheet and has been subjected to refining such as a solution heat treatment and a quenching treatment, but is not yet subjected to an artificial age hardening treatment such as a bake hardening treatment. Further, aluminum is hereinafter also referred to as Al.

BACKGROUND ART

In recent years, because of environmental awareness and the like, the society's requirement for weight reduction in a vehicle such as an automobile has been steadily increasing. In order to respond to such requirement, as a material for a large body panel (an outer panel or an inner panel) of an automobile panel such as a hood, a door, or a roof in particular, instead of a steel material such as a steel sheet, application of an aluminum alloy material excellent in formability and bake hardenability, and lighter in weight has been increasing.

Among these, for a panel such as an outer panel (outer sheet) or an inner panel (inner sheet) of a panel structure such as a hood, a fender, a door, a roof, or a trunk lid of an automobile, use of an Al—Mg—Si-based AA or JIS 6000 series (hereinafter, also simply referred to as a 6000 series) aluminum alloy sheet, as a thin and high strength aluminum alloy sheet, has been studied.

This 6000 series aluminum alloy sheet contains Si and Mg as essential elements, and particularly a 6000 series aluminum alloy with excess Si has a composition having 1 or greater Si/Mg mass ratio and has excellent age-hardenability. Therefore, it has bake hardenability (hereinafter, also referred to as bake hardenability=BH responses or bake hardenability) in which formability is secured by lowering the proof stress during press forming or bending, age hardening occurs by heating in an artificial aging (hardening) treatment at a comparatively low temperature, such as a bake hardening treatment of a panel after forming and the like, to improve the proof stress, whereby the strength required as a panel can be secured.

In the 6000 series aluminum alloy sheet, the alloy element amount is comparatively less in comparison to other 5000 series aluminum alloys and the like where alloy amounts such as Mg amount are large. Therefore, when the scrap of these 6000 series aluminum alloy sheets is reused as an aluminum alloy melting material (melting raw material), an original 6000 series aluminum alloy slab is easily obtained, and recycling performance is also excellent.

On the other hand, as is known well, an outer panel of an automobile is manufactured by applying combined forming work, such as stretch forming or bending forming in press forming, to an aluminum alloy sheet. For example, in a large outer panel such as a hood or a door, the shape of a formed product is made as an outer panel by press forming such as stretching, and then joining with an inner panel is executed by hem work (hemming) of a flat hem and the like of the outer panel peripheral section to be formed into a panel structural body.

Here, the 6000 series aluminum alloy had an advantage of having excellent BH responses, but had a problem of aging properties at room temperature, that is, of age hardening during retention at room temperature for several months after the solution heat treatment and quenching treatment to increase the strength, thereby deteriorating formability into a panel, particularly the bendability. For example, in a case where a 6000 series aluminum alloy sheet is to be used for an automobile panel, it is typically placed at room temperature (standing at room temperature) for approximately 1 to 4 months after the solution heat treatment and the quenching treatment (after manufacturing) at an aluminum manufacturer until forming work into a panel at an automobile manufacturer, and comes to be significantly age hardened (room temperature aged) during that time. Particularly, in the outer panel subjected to severe bending, there was such a problem that, although forming was possible without any problem after the lapse of 1 month from the manufacturing, cracking occurred in hem working after the lapse of 3 months. Therefore, in the 6000 series aluminum alloy sheet for an automobile panel particularly for an outer panel, it is necessary to suppress room temperature aging over a comparatively long period of approximately 1 to 4 months.

Moreover, in a case where such room temperature aging is great, a problem also occurs in that the BH responses deteriorate and the proof stress is not improved to the strength required as a panel by heating during an artificial aging (hardening) treatment at a comparatively low temperature, such as a bake treatment and the like of the panel after forming described above.

Hitherto, from viewpoints of the microstructure of the 6000 series aluminum alloy sheet and particularly the cluster (aggregate of atoms), various proposals have been made with respect to improvement of the BH responses and suppression of room temperature aging. However, in many of those, with respect to the present state of the cluster (aggregate of atoms) directly affecting the BH responses and aging properties at room temperature of the 6000 series aluminum alloy sheet, the behavior thereof was merely indirectly reasoned by analogy.

Meanwhile, a trial to directly measure and stipulate the cluster (aggregate of atoms) affecting the BH responses and aging properties at room temperature of the 6000 series aluminum alloy sheet has been made.

In Patent Document 1, out of the clusters (aggregates of atoms) observed in analysis of the microstructure of a 6000 series aluminum alloy sheet by using a transmission electron microscope at one million times, the average number density of the clusters whose circle equivalent diameter is within the range of 1 to 5 nm has been stipulated in the range of 4000 to 30000 pieces/μm² to obtain one with excellent BH responses and suppressed room temperature aging.

Moreover, Patent Documents 2 and 3 suggest that a 6000 series aluminum alloy sheet in which good BH responses can be exhibited, even when a vehicle body bake treatment after room temperature aging is performed, is obtained by controlling aggregates (clusters) of specific atoms directly measured by a three-dimensional atom probe field ion microscope. These Patent Documents describe aggregates of atoms which include either or both of Mg atoms and Si atoms by a total of 10 pieces or more or 30 pieces or more and, when using any atom of Mg atoms and Si atoms contained therein as a reference, the distance between the reference atom and any atom out of other atoms adjacent thereto is 0.75 nm or less.

In addition, Patent Document 2 describes that aggregates of atoms satisfying those conditions are contained at an average number density of 1.0×10⁵ pieces/μm³ or greater.

Further, Patent Document 3 describes that aggregates of atoms satisfying those conditions are contained at an average number density of 5.0×10²³ pieces/m³ or greater and aggregates of atoms having a size in which the radius of the circle equivalent diameter as the maximum is 1.5 nm or greater are contained such that, among the aggregates of atoms satisfying those conditions, the average number density of aggregates of atoms having a size in which the radius of the circle equivalent diameter as the maximum is less than 1.5 nm is regulated to be 10.0×10²³ pieces/m³ or less and a ratio a/b between an average number density a of aggregates of atoms having a size in which the radius of the circle equivalent diameter as the maximum is less than 1.5 nm and an average number density b of aggregates of atoms having a size in which the radius of the circle equivalent diameter as the maximum is 1.5 nm or greater is set to 3.5 or less.

Meanwhile, as prior patents relating to addition of Sn in the present invention, a plurality of methods of suppressing room temperature aging and improving bake hardening by actively adding Sn to a 6000 series aluminum alloy sheet have been suggested in addition to Patent Documents 4 and 5. For example, Patent Document 4 describes a method of combining room temperature aging suppression and bake hardening by restricting a component relationship between Mg and Si to “−2.0>4Mg-7Si,” adding an appropriate amount of Sn having an effect of suppressing a change over time, and performing pre-aging after the solution heat treatment. In addition, Patent Document 5 suggests a method of improving formability, baking finish properties, and corrosion resistance by restricting a component relationship between Mg and Si to “−2.0≦4Mg-7Si≦1.0,” adding Sn that has an effect of suppressing a change over time and Cu that improves formability, and performing zinc-based plating.

CITATION LIST Patent Documents

Patent Document 1: JP-A-2009-242904

Patent Document 2: JP-A-2012-193399

Patent Document 3: JP-A-2013-60627

Patent Document 4: JP-A-09-249950

Patent Document 5: JP-A-10-226894

SUMMARY OF INVENTION Technical Problem

However, demand for improved fuel efficiency of an automobile is still high and weight reduction is further progressed. In this manner, thickness reduction of an aluminum alloy sheet tends to be required. Meanwhile, in a conventional art in which the behavior of aggregates (clusters) of atoms is reasoned by analogy through indirect measurement or in Patent Document 1 in which the size and the number density of comparatively large aggregates of atoms which are evaluated by TEM observation are only controlled, the aggregates of atoms cannot be accurately or specifically evaluated. For this reason, the aggregates of atoms cannot be precisely controlled and the 13H responses after room temperature aging are insufficient. Further, in Patent Documents 2 and 3 in which aggregates (clusters) of specific atoms directly measured by a transmission electron microscope at one million times or a three-dimensional atom probe field ion microscope are controlled, there has been room for improvement of combining good BH responses after room temperature aging for a long period of time and preferable workability. The same applies to Patent Documents 4 and 5 in which Sn is actively added to a 6000 series aluminum alloy sheet.

In view of the above-described problems, an object of the present invention is to provide an Al—Si—Mg alloy sheet capable of exerting good BH responses and preferable workability, even in the vehicle body bake treatment after room temperature aging for a long period of time, by means of evaluating aggregates of atoms in a microstructure in more detail.

Solution to Problem

In order to achieve the object, the gist of an aluminum alloy sheet excellent in bake hardenability regarding an aspect of the present invention (hereinafter, also referred to as a first aspect of the present invention) is an Al—Mg—Si alloy sheet containing, in mass %, Mg: 0.2% to 2.0%, Si: 0.3% to 2.0% and Sn: 0.005% to 0.3%, with the remainder being Al and inevitable impurities, and containing an aggregate of atoms measured by a three-dimensional atom probe field ion microscope, in which either or both of an Mg atom and an Si atom are contained in the aggregate of atoms by a total of 10 pieces or more and, when any atom of the Mg atom and the Si atom contained therein is used as a reference, a distance between the atom as the reference and any atom among other atoms adjacent thereto is 0.75 nm or less, and in which the aggregate of atoms satisfying these conditions is contained in the aluminum alloy sheet at an average number density of 2.5×10²³ pieces/m³ or greater and 20.0×10²³ pieces/m³ or less, an average radius of a circle equivalent diameter of the aggregate of atoms satisfying these conditions is 1.15 nm or greater and 1.45 nm or less, and a standard deviation of the radius of the circle equivalent diameter is 0.45 nm or less.

Further, in order to achieve the object, the gist of an aluminum alloy sheet excellent in bake hardenability regarding another aspect of the present invention (hereinafter, also referred to as a second aspect of the present invention) is an Al—Mg—Si alloy sheet containing, in mass %, Mg: 0.2% to 2.0%, Si: 0.3% to 2.0% and Sn: 0.005% to 0.3%, with the remainder being Al and inevitable impurities, in which a ratio (N_(cluster)/N_(total))×100 of N_(cluster) to N_(total) is 1% or greater and 15% or less, in which the N_(total) represents a total number of all Mg atoms and Si atoms measured by a three-dimensional atom probe field ion microscope and the N_(cluster) represents a total number of all Mg atoms and Si atoms contained in all aggregates of atoms satisfying conditions in which an aggregate of atoms measured by the three-dimensional atom probe field ion microscope contains either or both of an Mg atom and an Si atom by a total of 10 pieces or more and, when any atom of the Mg atom and the Si atom is used as a reference, a distance between the atom as the reference and any atom among other atoms adjacent thereto is 0.75 nm or less, and an average radius of a circle equivalent diameter of the aggregate of atoms is 1.20 nm or greater and 1.50 nm or less.

Further, in order to achieve the object, the gist of an aluminum alloy sheet excellent in bake hardenability regarding another aspect of the present invention (hereinafter, also referred to as a third aspect of the present invention) is an Al—Mg—Si alloy sheet containing, in mass %, Mg: 0.2% to 2.0%, Si: 0.3% to 2.0% and Sn: 0.005% to 0.3%, with the remainder being Al and inevitable impurities, and containing an aggregate of atoms measured by a three-dimensional atom probe field ion microscope, in which either or both of an Mg atom and an Si atom are contained in the aggregate of atoms by a total of 10 pieces or more and, when any atom of the Mg atom and the Si atom contained therein is used as a reference, a distance between the atom as the reference and any atom among other atoms adjacent thereto is 0.75 nm or less, and in which the aggregate of atoms satisfying these conditions has an average number density of 3.0×10²³ pieces/m³ or greater and 25.0×10²³ pieces/m³ or less and, among the aggregates of atoms satisfying these conditions, an average proportion of an aggregate of atoms in which a ratio (Mg/Si) of a number of Mg atoms to a number of Si atoms is ½ or greater is 0.70 or greater.

Advantageous Effects of Invention

In the first aspect of the present invention, as for the Al—Mg—Si alloy sheet containing Sn, it was found that, among aggregates (clusters) of atoms measured by a 3DAP, the average number density of a specific cluster, which contains Mg atoms or Si atoms by a specific amount or greater in total as the stipulation described above and in which the distance between atoms adjacent to each other which are contained therein is a specific value or less, is greatly correlated with the BH responses.

Furthermore, according to the present embodiment, it was also found that the distribution state of the size of the aggregates of the specific atoms satisfying these conditions is important, and the average radius of the circle equivalent diameters and the standard deviation of radii of the circle equivalent diameters greatly affect the BH responses.

That is, in order to improve the BH responses of the Al—Mg—Si alloy sheet containing Sn, it was found to be required that the average radius of the circle equivalent diameters of aggregates of specific atoms is in a specific range of 1.15 nm or greater and 1.45 nm or less and the standard deviation has a small value of 0.45 nm or less for the improvement in BH responses. According to the present aspect, it is possible to provide an Al—Si—Mg alloy sheet which is capable of exhibiting better BH responses even in a case of being subjected to long-term room temperature aging for 100 days.

In the second aspect of the present invention, as for the Al—Mg—Si alloy sheet containing Sn, it is premised that, among aggregates (clusters) of atoms measured by a 3DAP, a great number of fine clusters in which the distance between the atoms is 0.75 nm or less are present. In addition, the total amount of Mg and Si present in these clusters is balanced with the total amount of Mg and Si which are made into a solid solution in a matrix and the distribution state of the size of the clusters is controlled as the average radius of the circle equivalent diameters of the clusters, thereby improving the BH responses.

When the total amount of Mg atoms and Si atoms present in a cluster stipulated as described above is secured after being balanced with the total amount of Mg and Si which are made into a solid solution in a matrix, the BH responses can be improved. Moreover, as an aspect other than being present in the cluster stipulated in the present aspect or being made into a solid solution in a matrix, there is a possibility that Mg and Si contained in the 6000 series aluminum alloy sheet are present by being contained in a cluster which is coarser than stipulated, or further coarser precipitate or an intermetallic compound. Meanwhile, when the total amount of Mg and Si present in the cluster is controlled after being balanced with the total amount of Mg and Si being made into a solid solution in a matrix, a coarse cluster originated by Mg and Si, and further coarser precipitate and an intermetallic compound can be reduced.

Further, since the distribution state of the size of the cluster greatly affects the BH responses, in order to improve the BH responses of the Al—Mg—Si alloy sheet containing Sn, it is necessary that the average radius of the circle equivalent diameters of aggregates of the atoms be adjusted to 1.20 nm or greater and 1.50 nm or less. By controlling the total amount of Mg atoms and Si atoms present in clusters and the distribution stated of the size of clusters, it is possible to provide an Al—Si—Mg alloy sheet which has excellent formability and is capable of exhibiting better BH responses even in a case of being subjected to long-term room temperature aging for 100 days.

In the third aspect of the present invention, as for the Al—Mg—Si alloy sheet containing Sn, it is premised that, among aggregates (clusters) of atoms measured by a 3DAP, a great number of fine clusters in which the distance between the atoms is 0.75 nm or less are present. Moreover, the BH responses can be improved by increasing the proportion of clusters with a great number of Mg atoms among elements constituting these fine clusters.

The present inventors found that even the same clusters have different influences on BH responses depending on the compositions thereof, and a cluster rich in Si atoms adversely affects BH responses, while a cluster rich in Mg atoms promotes the BH responses. For this reason, in the present aspect, among clusters measured by a 3DAP, the number of clusters in which the distance between the atoms is small is controlled to be great and among these clusters, the proportion of clusters having a large number of Mg atoms is controlled to be high to improve the BH responses.

In this manner, according to the present aspect, it is possible to provide an Al—Si—Mg alloy sheet which is capable of exhibiting better BH responses even in a case of room temperature aging.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be described in detail for each requirement.

Cluster (Aggregate of Atoms)

First, the meaning of a cluster referred to in the present invention will be described. Similar to Patent Documents 2 and 3 described above, the cluster in the present invention indicates an aggregate (cluster) of atoms measured by a 3DAP described below and this will be mainly expressed as a cluster in the description below. In a 6000 series aluminum alloy, it is known that Mg and Si form an aggregate of atoms which is referred to as a cluster, after a solution heat treatment and a quenching treatment, during retention at room temperature or a heat treatment at 50° C. to 150° C. In this case, the behaviors (properties) of clusters generated during retention at room temperature and during a heat treatment at 50° C. to 150° C. are completely different.

A cluster formed during retention at room temperature suppresses precipitation of a GP zone or a β′ phase that increases strength in the subsequent artificial aging or bake treatment. On the contrary, it is described that a cluster (alternatively, an Mg/Si cluster) formed at 50° C. to 150° C. promotes precipitation of a GP zone or a β′ phase (for example, described in Yamada et al., Journal of The Japan Institute of Light Metals, vol. 51, p. 215).

The paragraphs 0021 to 0025 of Patent Document 1 described above describe that these clusters are conventionally analyzed by specific heat measurement, a 3DAP (three-dimensional atom probe) or the like. At the same time, it also describes that even though the presence of a cluster itself could be confirmed through observation in the analysis of a cluster by using a 3DAP, the size or the number density of the cluster stipulated in the present invention was still unclear or could be only limitedly measured.

In the 6000 series aluminum alloy, the analysis of the cluster by using a 3DAP (three-dimensional atom probe) has been attempted from the past. However, as described in Patent Document 1, even though the presence of a cluster itself could be confirmed, the size or the number density of the cluster was unclear. This is because, among aggregates (clusters) of atoms measured by a 3DAP, which cluster being greatly correlated with BH responses was unclear, and which aggregate of atoms being greatly correlated with BH responses was unclear.

On the contrary, the present inventors clarified clusters greatly related to BH responses in Patent Document 2 described above. That is, it was found that, among clusters measured by a 3DAP, a specific cluster which contains Mg atoms or Si atoms by a specific amount or greater in total as stipulated above and in which the distance of atoms adjacent to each other which are contained therein is a specific value or less, is greatly correlated with BH responses. In addition, it was found that good BH responses can be exhibited by increasing the number density of an aggregate of atoms satisfying these conditions, even in a vehicle body bake treatment under the condition of a low temperature for a shortened time after room temperature aging.

According to Patent Document 2 described above, the presence of a cluster which contains either or both of Mg atoms and Si atoms by a total of 30 pieces or more, and in which the distance between atoms adjacent to each other is 0.75 nm or less improves BH responses. Further, it is described that an Al—Si—Mg alloy sheet after room temperature aging is capable of exhibiting better BH responses when a certain amount or greater of these clusters are present, even in a case of a vehicle body bake treatment at a low temperature for a shortened period of time of 150° C. for 20 minutes.

Meanwhile, as a result of further research, the present inventors found that the presence of a large amount of the clusters among clusters measured by a 3DAP certainly improves BH responses, but the effect of improving the BH responses is not sufficient only with that. In other words, it was found that the presence of a large amount of the clusters is a precondition (necessary condition) of improvement of BH responses, but is not necessarily a sufficient condition.

For this reason, the present inventors further filed Patent Document 3 described above. This is because they found that clusters containing either or both of Mg atoms and Si atoms have understandably a difference (distribution) in size thereof, and thus, there is a great difference in action on the BH responses depending on the size of the clusters. In other words, the actions on BH responses depending on the size of the clusters are opposite, that is, a cluster with a comparatively small size inhibits BH responses and a cluster with a comparatively large size promotes BH responses. Based on this, the BH responses can be further improved by reducing clusters with a comparatively small size and increasing clusters with a comparatively large size among the above-described specific clusters. It is considered that even though a cluster with a comparatively small size disappears during a BH treatment (during artificial age hardening treatment), it rather inhibits precipitation, during this BH, of a large cluster which greatly effects in improvement of strength, thereby degrading the BH responses. Meanwhile, it is considered that a cluster with comparatively large size grows during the BH treatment and promotes precipitation of precipitates during the BH treatment, thereby improving the BH responses.

It was found that when an extremely large cluster grows during the BH treatment, the size thereof becomes extremely large and this rather leads to degradation of the BH responses and to an extreme increase in the strength before the BH treatment, and thus the workability is deteriorated. That is, the clusters having an optimal size are present in order to improve the BH responses without deteriorating the workability. It was also found that, even though the distribution state of the size of aggregates of the specific atoms is important, the average radius of the circle equivalent diameters, which is an average size of aggregates of these specific atoms and standard deviation of radii of the circle equivalent diameters, greatly affects the BH responses. The present inventors further filed these contents as an application of Japanese Patent Application No. 2012-051821 (filed on Mar. 8, 2012). In Japanese Patent Application No. 2012-051821, only clusters having an optimal size are generated by setting the average radius of the circle equivalent diameters of clusters to 1.2 nm or greater and 1.5 nm or less and setting the standard deviation of radii of the circle equivalent diameters to 0.35 nm or less.

Based on the subsequent research, related to the first embodiment of the present invention, it has been found that optimal ranges, which improve the BH responses, of the average radius of the circle equivalent diameters which is an average size of aggregates of the specific atoms and the standard deviation of radii of the circle equivalent diameters in an Al—Mg—Si alloy sheet containing Sn are different from those of an Al—Mg—Si alloy sheet of the prior application which does not contain Sn. That is, in order to improve the BH responses of the Al—Mg—Si alloy sheet containing Sn, it has been found to be required that the average radius of the circle equivalent diameters of aggregates of the specific atoms is in a specific range of 1.15 nm or greater and 1.45 nm or less and the standard deviation of radii of the circle equivalent diameters has a small value of 0.45 nm or less for the improvement in BH responses. In order to improve the BH responses, it is preferable that only the clusters having an optimal size are generated, not that the size of aggregates of the specific atoms is in a broad range from a small value to a great value and unevenness in size distribution is large. This is intended by that the average radius of the circle equivalent diameters is 1.15 nm or greater and 1.45 nm or less and the standard deviation of the radii of the circle equivalent diameters is 0.45 nm or less as stipulated in the first embodiment of the present invention. In this manner, in the first embodiment of the present invention, it is possible to further improve the BH responses of the Al—Mg—Si alloy sheet, even in a case of long-term retention at room temperature for 100 days with a vehicle body bake treatment being performed.

Moreover, related to the second embodiment of the present invention, it has been found that, in the Al—Mg—Si alloy sheet containing Sn, the balance between the aggregate (cluster) of atoms and the amount of Mg atoms and Si atoms made into a solid solution greatly affects the BH responses and the strength after the BH treatment. That is, the second embodiment of the present invention is based on the knowledge that the strength before the baking finish becomes higher and the BH responses can be improved by controlling the proportions of Mg atoms and Si atoms contained in the aggregates of atoms satisfying the stipulated conditions and Mg and Si present in a matrix.

Further, it has been found that, in the Al—Mg—Si alloy sheet containing Sn, when an extremely large cluster grows during the BH treatment, the size thereof becomes extremely large and this rather leads to degradation of the BH responses and to an extreme increase in the strength before the BH treatment, and thus the workability is deteriorated. That is, it also has been found that clusters having an optimal size are present in order to improve the BH responses without deteriorating the workability. It also has been found that, even though the distribution state of the size of aggregates of the specific atoms is important, the average radius of the circle equivalent diameters which is an average size of aggregates of these specific atoms greatly affects the BH responses. That is, in order to improve the BH responses of the Al—Mg—Si alloy sheet containing Sn, it is required that the average radius of the circle equivalent diameters of aggregates of the specific atoms is in a specific range of 1.20 nm or greater and 1.50 nm or less for the improvement of BH responses.

Further, related to the third embodiment of the present invention, it has been found that, in the Al—Mg—Si alloy sheet containing Sn, even the same clusters have different influences on BH responses as described above depending on the compositions thereof, and a cluster rich in Si atoms adversely affects BH responses, while a cluster rich in Mg atoms promotes the BH responses. This is the concept of the present embodiment and, for this reason, in the present embodiment, among clusters measured by a 3DAP, the number of clusters in which the distance between the atoms is small is controlled to be great and among these clusters, the proportion of clusters having a great number of Mg atoms is controlled to be large, whereby the BH responses can be improved.

Cluster in First Embodiment of the Present Invention

Hereinafter, the cluster in the first embodiment of the present invention will be described.

Stipulation of Cluster of Present Embodiment

Hereinafter, stipulation of the cluster of the present embodiment will be described in detail.

The aluminum alloy sheet in which the cluster is stipulated in the present embodiment indicates, as described above, a sheet after a series of refining such as a solution heat treatment, a quenching treatment and a re-heating treatment are applied thereto after rolling and indicates a sheet before being subjected to forming work into a panel by press forming or the like (a sheet before being subjected to an artificial age hardening treatment such as a bake hardening treatment). In this case, in order to be press-formed as the above-described automobile panel or the like, it is likely to stand at room temperature for a comparatively long period of time, that is, approximately 1 month to 4 months after production of the sheet. For this reason, it is preferable that the microstructure is as stipulated in the present embodiment, even in a case of a microstructure of the sheet after standing at room temperature for a long period of time. From this viewpoint, in a case where the characteristics after long-term room temperature aging are issues, since it is assumed that the characteristics are not changed and the microstructure is not changed after room temperature aging for approximately 100 days, it is more preferable that the microstructure and characteristics of a sheet after room temperature aging has sufficiently advanced, that is, after the above-described series of refining have been carried out, and then 100 or more days have passed are examined and evaluated.

Definition of Cluster of the Present Embodiment

The microstructure in an arbitrary center part in the thickness direction of the Al—Mg—Si alloy sheet after being subjected to refining such as the solution heat treatment or the quenching treatment before standing at room temperature is measured by a three-dimensional atom probe field ion microscope. As the cluster present in the measured microstructure, according to the present embodiment, first, the cluster contains either or both of Mg atoms and Si atoms by a total of 10 pieces or more. Further, the number of pieces of the Mg atoms or the Si atoms contained in the aggregate of atoms is preferably as large as possible and the upper limit thereof is not particularly limited. However, from the manufacturing limit, the upper limit of the number of pieces of Mg atoms and Si atoms contained in the cluster is approximately 10000 pieces.

In Patent Document 2 described above, the cluster contains either or both of Mg atoms and Si atoms by a total of 30 pieces or more. However, in the present embodiment, as described above, since clusters with comparatively small size inhibit BH responses, they are regulated so that the number thereof becomes small. For this reason, in order to control the clusters with comparatively small size which need to be regulated to be in a measurable range, it is stipulated that either or both of Mg atoms and Si atoms are contained in a total of 10 pieces or more, similar to Patent Document 3 described above.

Further in the present embodiment, similar to Patent Documents 2 and 3 described above, one in which, when any atom of the Mg atoms and the Si atoms contained in the cluster is used as a reference, the distance between the atom as the reference and any atom among other atoms adjacent thereto is 0.75 nm or less is set as an aggregate (cluster) of atoms stipulated in the present embodiment (satisfying the stipulation of the present embodiment). The distance therebetween of 0.75 nm is a numerical value determined in order to assure the number density of the cluster with a large size in which the distance between atoms such as Mg and Si is short and which is effective in improving BH responses at a low temperature in a short period of time after long-term room temperature aging, and to regulate the clusters with a small size to control the number density thereof to be small. The present inventors have hitherto performed an intensive research on the relationship between the aluminum alloy sheet capable of exerting excellent BH responses even in the vehicle body bake treatment under the conditions of a low temperature for a shortened time and the aggregate of an atomic level. As a result, it has been found through experimentation that the high number density of the aggregates of atoms stipulated by the definition described above represents the form of the microstructure exerting good BH responses. Therefore, although the technical implication of the distance of 0.75 nm between atoms has not been sufficiently clarified, it is important for the purpose of strictly assuring the number density of an aggregate of atoms exerting good BH responses and is a numerical value determined for the purpose.

The cluster stipulated in the present embodiment contains both of Mg atoms and Si atoms in most cases. However, the cases where Mg atoms are contained but Si atoms are not contained, or the cases where Si atoms are contained but Mg atoms are not contained are involved. Further, it is not constantly configured only of the Mg atoms and the Si atoms, and there is a high probability that Al atoms are additionally contained.

Moreover, depending on the component composition of the Al—Mg—Si alloy sheet containing Sn which is the object of the present embodiment, a case inevitably exists, in which atoms such as Sn, Fe, Mn, Cu, Cr, Zr, V, Ti, Zn, or Ag contained as alloy elements or impurities, are contained in the cluster and these other atoms are counted by 3DAP analysis. However, even when these other atoms (derived from alloy elements or impurities) are contained in the cluster, these are lower levels than the total number of Mg atoms and Si atoms. Therefore, even in a case where such other atoms are contained in the cluster, those satisfying the stipulation (condition) function as the cluster of the present embodiment in the same manner as in the cluster formed of only Mg atoms and Si atoms. Accordingly, the cluster stipulated in the present embodiment may contain any other atoms, as long as the stipulation described above is satisfied.

In addition, the expression “when any atom of Mg atoms and Si atoms contained therein is used as a reference, the distance between the atom as the reference and any atom among other atoms adjacent thereto is 0.75 mm or less” of the present embodiment means that all of the Mg atoms and the Si atoms present in the cluster contain, in the periphery thereof, at least one Mg atom or Si atom with the distance therebetween being 0.75 nm or less.

With respect to the stipulation on the distance between atoms in the cluster of the present embodiment, when any atom of Mg atoms and Si atoms contained therein is used as a reference, all distances between the atom as the reference and all atoms of other atoms adjacent thereto are not necessarily 0.75 nm or less. On the contrary, all of them may be 0.75 nm or less. In other words, other Mg atoms or Si atoms whose distance therebetween exceeds 0.75 nm may be adjacent to each other, and at least one of other Mg atoms and Si atoms satisfying the stipulated distance (space) may be present in the periphery of a specific (serving as a reference) Mg atom or Si atom.

In a case where one other Mg atom or Si atom adjacent to the reference atom, which satisfies the stipulated distance is present, the number of Mg atoms or Si atoms to be counted, that satisfy the condition of the distance is 2 including the specific (serving as a reference) Mg atom or Si atom. In the case where two other Mg atoms or Si atoms adjacent to the reference atom, which satisfy the stipulated distance is present, the number of Mg atoms or Si atoms to be counted, that satisfy the condition of the distance is 3 including the specific (serving as a reference) Mg atom or Si atom.

The cluster described above is a cluster generated by a temperature holding treatment after a solution heat treatment and a stopping of a quenching treatment at a high temperature in refining after rolling described above and below in detail. That is, the cluster of the present embodiment is an aggregate of atoms generated by a temperature holding treatment after a solution heat treatment and a stopping of a quenching treatment at a high temperature, and is a cluster which contains either or both of Mg atoms and Si atoms by a total of 10 pieces or more and, when any atom of Mg atoms and Si atoms contained therein is used as a reference, the distance between the atom as the reference and any atom among other atoms adjacent thereto is 0.75 nm or less.

Until now, it has been reported that a cluster promoting precipitation of a GP zone or a β′ phase that increases the strength in artificial aging or a bake treatment is a Mg/Si cluster as described above, and this cluster is formed by a heat treatment at 50° C. to 150° C. after the solution heat treatment and the quenching treatment. Meanwhile, a cluster that suppresses precipitation of a GP zone or a β′ phase in the artificial aging treatment or the bake treatment is an Si-rich cluster and the cluster is formed by retention at room temperature (room temperature aging) after the solution heat treatment and the quenching treatment (for example, described in Sato, Journal of The Japan Institute of Light Metals, vol. 56, p. 595).

However, as a result of the detailed analysis by the present inventors on the relationship between the strength during the artificial aging treatment or during the bake treatment and the cluster, it has been found that a structural factor contributing to the strength during the artificial aging treatment or during the bake treatment is not the kind (composition) of the cluster but the distribution state of the size of the cluster generated by the refining treatment of a sheet. Further, correspondence of the distribution state of the size of the clusters to the strength during the artificial aging treatment or during the bake treatment has been also clarified only by the analysis with the definition described above.

On the other hand, the clusters formed by retention at room temperature (room temperature aging) have the number of atoms and the number density of the clusters deviating from the stipulation of the present embodiment although they are the aggregates of atoms in measurement by a three-dimensional atom probe field ion microscope. Therefore, the stipulation of the clusters (aggregates of atoms) of the present embodiment is also a stipulation distinguishing the clusters from those formed by retention at room temperature (room temperature aging) described above and preventing added (contained) Mg and Si from being consumed by these clusters.

(Density of Cluster)

The cluster defined in the present embodiment or the cluster satisfying the preconditions described above is contained in the average number density of 2.5×10²³ pieces/m³ or greater and 20.0×10²³ pieces/m³ or less. When the average number density of the cluster is much less than 2.5×10²³ pieces/m³, clusters which are extremely small are newly generated during room temperature aging for a long period of time, and thus, degradation of BH responses and deterioration of workability is caused. Meanwhile, when it is much greater than 20.0×10²³ pieces/m³, the strength before the BH treatment becomes extremely high, and thus the workability is deteriorated.

When the average number density of the clusters stipulated in the present embodiment is small, the formation amount of the clusters themselves becomes insufficient, which means that much of added (contained) Mg and Si are consumed by the clusters formed by the room temperature aging described above. For this reason, even though there may be an effect of promoting precipitation of a GP zone or a phase and improving the BH responses, after standing at room temperature (room temperature aging) for a long period of time, improvement of the BH responses remains conventional approximately 30 MPa to 40 MPa in terms of 0.2% proof stress. Accordingly, desired better BH responses cannot be secured under such conditions.

Stipulation of Size Distribution of Clusters of the Present Embodiment

On the premise that the above-described predetermined amount (equal to or greater than the average number density) of the cluster defined in the present embodiment is allowed to be present, in the present embodiment, in order to improve the BH responses, the average radius of the circle equivalent diameters of the aggregates of atoms satisfying the conditions is 1.15 nm or greater and 1.45 nm or less, and the standard deviation of radii of the circle equivalent diameters is 0.45 nm or less, as described above.

Average Radius E(r) of Circle Equivalent Diameters of Aggregates of Atoms

An average radius E(r) (nm) of the circle equivalent diameters of the aggregates of atoms satisfying the preconditions described above is represented by “E(r)=(1/n)Σr.” Here, n represents the number of aggregates of atoms satisfying the preconditions. r represents the radii (nm) of circle equivalent diameters of aggregates of respective atoms satisfying the preconditions.

First, the size itself of the aggregates of atoms satisfying the above-described preconditions is important for improving the BH responses. The aggregates (clusters) of atoms whose average radius E(r) of the circle equivalent diameters is extremely small disappear during the BH treatment (during artificial aging hardening treatment) and suppress precipitation of intermediate precipitates such as β″ or β′ which are highly effective for improving the strength during the BH treatment, whereby inhibit the BH responses. Meanwhile, the aggregates (clusters) of atoms whose average radius E(r) of the circle equivalent diameters is extremely large already precipitate as intermediate precipitates such as β″ or β′ due to room temperature aging at the time before the BH treatment (previously or in advance) and rather increase the strength before the BH treatment, whereby inhibit the press formability or bendability. When it already precipitate as intermediate precipitates such as β″ or (V at the time before the BH treatment, this suppresses the precipitation of the intermediate precipitates such as a new β″ or β′ during the BH and also leads to inhibition the BH responses. In addition, β″ and β′ are both intermediate precipitated phases and are both Mg₂Si. However, since the particle structures (arrangement of atoms) thereof are different from each other and it is difficult to use separate expressions for them, in a case where “′ (apostrophe)” cannot be used, β′ is referred to as a β prime and β″ is referred to as a β double prime.

Meanwhile, an aggregate of atoms satisfying the preconditions as described above, and the size thereof being in a range of 1.15 nm or greater and 1.45 nm or less by the average radius E(r) of the circle equivalent diameter precipitates an intermediate precipitate such as β″ or β′ which is highly effective for improving the strength (which contributes to improvement of the strength) during the BH treatment. Therefore, it can have characteristics in which the strength is low and workability is good at the stage of press forming or bending and the strength becomes high initially after the BH treatment. For this reason, the size of the stipulated aggregate of atoms is set such that the average radius E(r) of the circle equivalent diameter is 1.15 nm or greater and 1.45 nm or less.

Standard deviation a of circle equivalent diameters of aggregates of atoms The standard deviation σ of the circle equivalent diameters of aggregates of atoms satisfying the preconditions described above is represented by “σ²=(1/n)Σ[r−E(r)]²” from the average radius E(r) of the circle equivalent diameters.

The size of the aggregates of atoms satisfying the preconditions described above, that is, the average radius E(r) of the circle equivalent diameters is important, but the standard deviation of the average radius E(r) of the circle equivalent diameters which is the average size of the aggregates of specific atoms satisfying the preconditions described above greatly affects the BH responses as the distribution state of the size of the aggregates of atoms. In other words, in order to improve the BH responses, it is required that the average radius of the circle equivalent diameters of the aggregates of specific atoms is in the specific range of 1.15 nm or greater and 1.45 nm or less and the standard deviation of the radii of the circle equivalent diameters has a small value of 0.45 nm or less for the improvement of the BH responses.

In order to improve the BH responses, it is preferable that only the clusters having an optimal size are generated, not that the size of aggregates of the specific atoms is greatly uneven from a small value to a great value. Such the state where only the clusters having an optimal size are generated is intended by that the average radius of the circle equivalent diameters is 1.15 nm or greater and 1.45 nm or less and the standard deviation of the radii of the circle equivalent diameters is 0.45 nm or less as described above. In this manner, in the present embodiment, it is possible to further improve the BH responses of the Al—Mg—Si alloy sheet, even in a case where a vehicle body bake treatment is performed after long-term retention at room temperature.

In the present embodiment, the size distribution of the stipulated aggregates of atoms is stipulated by both of the average radius of the circle equivalent diameters of the stipulated aggregates of atoms and the standard deviation of the radii of the circle equivalent diameters, whereby the number or the proportion of the aggregates (clusters) of atoms having a similar size among the stipulated aggregates of atoms is increased. In this manner, the BH responses of the Al—Mg—Si alloy sheet are further improved, even in a case where a vehicle body bake treatment is performed after long-term retention at room temperature.

Even in a case of the clusters stipulated in the above, when the number of clusters having a small size which inhibit the BH responses is large, the average radius of the circle equivalent diameters among the stipulations above becomes less than 1.15 nm, which is small. In addition, the standard deviation of the radii of the circle equivalent diameters exceeds 0.45 nm, which is large.

Meanwhile, even in a case of the clusters stipulated in the present embodiment, when the number of clusters having a large size which inhibit the BH responses is large, the average radius of the circle equivalent diameters among the stipulations above exceeds 1.45 nm, which is large. In addition, the standard deviation of the radii of the circle equivalent diameters exceeds 0.45 nm, which is large.

The aggregates (clusters) of atoms whose average radius of the circle equivalent diameters is extremely small disappear during the BH treatment (during artificial aging hardening treatment) and suppress precipitation of intermediate precipitates such as β″ or β′ which are highly effective for improving the strength (contributing to improvement of strength) during the BH treatment, whereby inhibit the BH responses. Meanwhile, the aggregates (clusters) of atoms whose average radius of the circle equivalent diameters is extremely large already precipitate as intermediate precipitates such as β″ or β′ due to room temperature aging at the time before the BH treatment and increase the strength before the BH treatment, whereby inhibit the bendability. When it already precipitate as intermediate precipitates such as β″ or β′ at the time before the BH treatment, this suppresses the precipitation of the intermediate precipitates such as a new β″ or β′ during the BH and also leads to inhibition the BH responses.

Cluster in Second Embodiment of the Present Invention

Hereinafter, the cluster in the second embodiment of the present invention will be described.

Stipulation of Cluster of Present Embodiment

Hereinafter, stipulation of the cluster of the present embodiment will be described in detail.

The aluminum alloy sheet in which the cluster is stipulated in the present embodiment indicates, as described above, a rolled sheet such as a hot rolled sheet or a cold rolled sheet after refining such as a solution heat treatment and a quenching treatment is applied thereto and indicates a sheet before being subjected to forming work into a panel by press forming or the like (a sheet before being subjected to an artificial age hardening treatment such as a bake hardening treatment). In this case, in order to be formed as the above-described automobile member or the like, it is likely to stand at room temperature for a comparatively long period of time, that is, approximately 0.5 month to 4 months after production of the sheet. For this reason, it is preferable that the microstructure is as stipulated in the present embodiment, even in a case of a microstructure of the sheet after standing at room temperature for a long period of time. From this viewpoint, in a case where the characteristics after long-term room temperature aging are issues, since it is assumed that the characteristics are not changed and the microstructure is not changed after room temperature aging for approximately 100 days, it is more preferable that the microstructure and characteristics of a sheet after room temperature aging has sufficiently advanced, that is, after the above-described series of refining have been carried out, and then 100 or more days have passed are examined and evaluated.

Definition of Cluster of the Present Embodiment

The microstructure in an arbitrary center part in the thickness direction of such an aluminum alloy sheet is measured by a three-dimensional atom probe field ion microscope. As the cluster present in the measured microstructure, according to the present embodiment, first, the cluster contains either or both of Mg atoms and Si atoms by a total of 10 pieces or more. Further, the number of pieces of the Mg atoms or the Si atoms contained in the aggregate of atoms is preferably as large as possible and the upper limit thereof is not particularly limited. However, from the manufacturing limit, the upper limit of the number of pieces of Mg atoms and Si atoms contained in the cluster is approximately 10000 pieces.

In Patent Document 2 described above, the cluster contains either or both of Mg atoms and Si atoms by a total of 30 pieces or more. However, in the present embodiment, as described above, since clusters with comparatively small size inhibit BH responses, they are regulated so that the number thereof becomes small. For this reason, in order to control the clusters with comparatively small size which need to be regulated to be in a measurable range, it is stipulated that either or both of Mg atoms and Si atoms are contained in a total of 10 pieces or more, similar to Patent Document 3 described above.

Further in the present embodiment, similar to Patent Documents 2 and 3 described above, one in which, when any atom of the Mg atoms and the Si atoms contained in the cluster is used as a reference, the distance between the atom as the reference and any atom among other atoms adjacent thereto is 0.75 nm or less is set as an aggregate (cluster) of atoms stipulated in the present embodiment (satisfying the stipulation of the present embodiment). The distance therebetween of 0.75 nm is a numerical value determined in order to assure the number density of the cluster with a large size in which the distance between atoms such as Mg and Si is short and which is effective in improving BH responses after long-term room temperature aging, and to regulate the clusters with a small size to control the number density thereof to be small. The present inventors have hitherto performed an intensive research on the relationship between the aluminum alloy sheet capable of exerting excellent BH responses even in the vehicle body bake treatment and the aggregate of an atomic level. As a result, it has been found through experimentation that the high number density of the aggregates of atoms stipulated by the definition described above represents the form of the microstructure exerting good BH responses. Therefore, although the technical implication of the distance of 0.75 nm between atoms has not been sufficiently clarified, it is important for the purpose of strictly assuring the number density of an aggregate of atoms exerting good BH responses and is a numerical value determined for the purpose.

The cluster stipulated in the present embodiment contains both of Mg atoms and Si atoms in most cases. However, the cases where Mg atoms are contained but Si atoms are not contained, or the cases where Si atoms are contained but Mg atoms are not contained are involved. Further, it is not constantly configured only of the Mg atoms and the Si atoms, and there is a high probability that Al atoms are additionally contained.

Moreover, depending on the component composition of the Al—Mg—Si alloy sheet containing Sn which is the object of the present embodiment, a case inevitably exists, in which atoms such as Sn, Fe, Mn, Cu, Cr, Zr, V, Ti, Zn, or Ag contained as alloy elements or impurities, are contained in the cluster and these other atoms are counted by 3DAP analysis. However, even when these other atoms (derived from alloy elements or impurities) are contained in the cluster, these are lower levels than the total number of Mg atoms and Si atoms. Therefore, even in a case where such other atoms are contained in the cluster, those satisfying the stipulation (condition) function as the cluster of the present embodiment in the same manner as in the cluster formed of only Mg atoms and Si atoms. Accordingly, the cluster stipulated in the present embodiment may contain any other atoms, as long as the stipulation described above is satisfied.

In addition, the expression “when any atom of Mg atoms and Si atoms contained therein is used as a reference, the distance between the atom as the reference and any atom among other atoms adjacent thereto is 0.75 mm or less” of the present embodiment means that all of the Mg atoms and the Si atoms present in the cluster contain, in the periphery thereof, at least one Mg atom or Si atom with the distance therebetween being 0.75 nm or less.

With respect to the stipulation on the distance between atoms in the cluster of the present embodiment, when any atom of Mg atoms and Si atoms contained therein is used as a reference, all distances between the atom as the reference and all atoms of other atoms adjacent thereto are not necessarily 0.75 nm or less. On the contrary, all of them may be 0.75 nm or less. In other words, other Mg atoms or Si atoms whose distance therebetween exceeds 0.75 nm may be adjacent to each other, and at least one of other Mg atoms and Si atoms satisfying the stipulated distance (space) may be present in the periphery of a specific (serving as a reference) Mg atom or Si atom.

In a case where one other Mg atom or Si atom adjacent to the reference atom, which satisfies the stipulated distance is present, the number of Mg atoms or Si atoms to be counted, that satisfy the condition of the distance is 2 including the specific (serving as a reference) Mg atom or Si atom. In the case where two other Mg atoms or Si atoms adjacent to the reference atom, which satisfy the stipulated distance is present, the number of Mg atoms or Si atoms to be counted, that satisfy the condition of the distance is 3 including the specific (serving as a reference) Mg atom or Si atom.

The cluster described above is a cluster generated by a temperature holding treatment after a solution heat treatment and a stopping of a quenching treatment at a high temperature in refining after rolling described above and below in detail. That is, the cluster of the present embodiment is an aggregate of atoms generated by a temperature holding treatment after a solution heat treatment and a stopping of a quenching treatment at a high temperature, and is a cluster which contains either or both of Mg atoms and Si atoms by a total of 10 pieces or more and, when any atom of Mg atoms and Si atoms contained therein is used as a reference, the distance between the atom as the reference and any atom among other atoms adjacent thereto is 0.75 nm or less.

(Amount of Mg and Si in Cluster)

In the cluster defined in the above-described manner (satisfying the preconditions) in the present embodiment, the total amount of Mg atoms and Si atoms which are present in all the clusters contained in the entirety of the Al—Mg—Si alloy sheet containing Sn is controlled with regard to the total amount of Mg and Si contained in the entirety of the aluminum alloy sheet. This means that the balance between the total amount of Mg atoms and Si atoms present in the defined clusters and the total amount of Mg atoms and Si atoms made into a solid solution in a matrix of the aluminum alloy sheet is suitably controlled. In this manner, the BH responses can be improved.

For the purposes of controlling the balance, in the present embodiment, on the premise that the measurement is performed by using a three-dimensional atom probe field ion microscope, the ratio of N_(cluster) which is the total of all pieces measured (total amount) of Mg atoms and Si atoms contained in specific clusters (aggregates of atoms) to N_(total) which is the total of all measured pieces of Mg atoms and Si atoms is set to be a predetermined value.

That is, the ratio (N_(cluster)/N_(total))×100 of N_(cluster) to N_(total) is set to fall within a range of 1% or greater and 15% or less. Here, the ratio of N_(cluster) to N_(total) calculated by (N_(cluster)/N_(total))×100 is the average (average ratio) of the values in a plurality of measurement sites of the center part in the thickness direction of a test sheet in terms of reproducibility as described in Examples below.

By obtaining such a well-balanced microstructure, after 100 days of the retention at room temperature (standing at room temperature) after a sheet is produced, the strength after the bake treatment being 200 MPa or greater and the BH responses (difference in strength between before and after the bake treatment) exceeding 90 MPa can be realized.

However, such the fact of the correlation between the microstructure and the BH responses is merely found experimentally and the mechanism thereof has not been sufficiently elucidated yet. When the average ratio (N_(cluster)/N_(total))×100 of N_(cluster) to N_(total) is less than 1%, the amount of Mg and Si made into a solid solution in the aluminum alloy sheet becomes larger and thus precipitation strengthening due to the clusters becomes weak and the strength before the bake treatment becomes weak due to the limitation of strengthening of the solid solution. For this reason, the strength after the bake treatment is likely to be weak inevitably.

In addition, in a case where the average ratio (N_(cluster)/N_(total))×100 of N_(cluster) to N_(total) exceeds 15%, the amount of Mg and Si contained in the cluster becomes extremely large and the Mg and Si made into a solid solution in the aluminum alloy sheet becomes small. For this reason, the number of strengthening phases or) generated during the artificial aging hardening treatment is decreased and thus the BH responses are likely to be degraded and the strength after the bake treatment is likely to be weak.

(Density of Cluster)

In order to control the average ratio (N_(cluster)/N_(total))×100 of N_(cluster) to N_(total) to be in a range of 1% to 15%, it is preferable that the clusters stipulated in the present embodiment are contained at an average number density of 2.5×10²³ pieces/m³ or greater. When the average number density of the clusters is smaller than 2.5×10²³ pieces/m³, the formation amount of the clusters themselves becomes insufficient which means that plenty of added (contained) Mg and Si are consumed by the clusters formed by the room temperature aging described above. For this reason, it becomes difficult to control the total amount of Mg and Si present in the clusters to be 1% or greater and thus the effect of improving the BH responses is deteriorated after standing at room temperature (room temperature aging) for a long period of time. Further, the preferred range of the average number density of the clusters is the average number density range of 2.5×10²³ pieces/m³ or greater and 20.0×10²³ pieces/m³ or less.

Stipulation of Size Distribution of Clusters of Present Embodiment

In the Al—Mg—Si alloy sheet containing Sn of the present embodiment, the total amount of atoms of Mg and Si in the clusters is controlled and the average radius of the circle equivalent diameters of the aggregates of the atoms satisfying the conditions is controlled to be 1.20 nm or greater and 1.50 nm or less. A cluster whose size in average radius E(r) of the circle equivalent diameter falls within a range of 1.20 nm or greater and 1.50 nm or less among the clusters in which the total amount of Mg atoms and Si atoms is controlled precipitates as an intermediate precipitate such as β″ or β′ which is highly effective for improving the strength (which contributes to improvement of the strength) during the BH treatment. Therefore, the characteristics can be obtained, in which the strength is low and workability is good at the stage of press forming or bending and the strength becomes high initially after the BH treatment.

An average radius E(r) (nm) of the circle equivalent diameters of the aggregates of atoms is represented by “E(r)=(1/n)Σr.” Here, n represents the number of aggregates of atoms satisfying the preconditions. r represents the radii (nm) of circle equivalent diameters of aggregates of respective atoms satisfying the preconditions.

The aggregates (clusters) of atoms whose average radius E(r) of the circle equivalent diameters is extremely small disappear during the BH treatment (during artificial aging hardening treatment) and suppress precipitation of intermediate precipitates such as β″ or β′ which are highly effective for improving the strength during the BH treatment, whereby inhibit the BH responses. Meanwhile, the aggregates (clusters) of atoms whose average radius E(r) of the circle equivalent diameters is extremely large already precipitate as intermediate precipitates such as β″ or β′ due to room temperature aging at the time before the BH treatment (previously or in advance) and rather increase the strength before the BH treatment, whereby inhibit the press formability or bendability. When it already precipitate as intermediate precipitates such as β″ or β′ at the time before the BH treatment, this suppresses the precipitation of the intermediate precipitates such as a new β″ or β′ during the BH and also leads to inhibition the BH responses. In addition, β″ and β′ are both intermediate precipitated phases and are both Mg₂Si. However, since the particle structures (arrangement of atoms) thereof are different from each other and it is difficult to use separate expressions for them, in a case where “′ (apostrophe)” cannot be used, β′ is referred to as a β prime and β″ is referred to as a β double prime.

As described above, even in a case of clusters in which the total amount of Mg atoms and Si atoms is controlled, when the number of clusters having a small size which inhibit the BH responses is large, the average radius of the circle equivalent diameters becomes less than 1.20 nm, which is small, whereby the BH responses are degraded. Further, even in a case of clusters stipulated in the present embodiment, when the number of clusters having a large size which inhibit the BH responses is large, the average radius of the circle equivalent diameters exceeds 1.50 nm, which is great, in the stipulated clusters. Therefore, the strength before the BH treatment becomes extremely high, the press formability or bendability is degraded, and then BH responses are degraded.

Meanwhile, an aggregate of atoms satisfying the preconditions as described above, and the size thereof being in a range of 1.20 nm or greater and 1.50 nm or less by the average radius E(r) of the circle equivalent diameter precipitates an intermediate precipitate such as β″ or β′ which is highly effective for improving the strength (which contributes to improvement of the strength) during the BH treatment. Therefore, it can have characteristics in which the strength is low and workability is good at the stage of press forming or bending and the strength becomes high initially after the BH treatment.

Cluster in Third Embodiment of the Present Invention

Hereinafter, the cluster in the third embodiment of the present invention will be described.

Stipulation of Cluster of Present Embodiment

Hereinafter, stipulation of the cluster of the present embodiment will be described in detail.

The aluminum alloy sheet in which the cluster is stipulated in the present embodiment indicates, as described above, a sheet after a series of refining such as a solution heat treatment, a quenching treatment and a re-heating treatment are applied thereto after rolling and indicates a sheet before being subjected to forming work into a panel by press forming or the like (a sheet before being subjected to an artificial age hardening treatment such as a bake hardening treatment).

In this case, in order to be press-formed as the above-described automobile panel or the like, it is likely to stand at room temperature for a comparatively long period of time, that is, approximately 1 month to 4 months after production of the sheet. For this reason, it is preferable that the microstructure is as stipulated in the present embodiment, even in a case of a microstructure of the sheet after standing at room temperature for a long period of time. From this viewpoint, in a case where the characteristics after long-term room temperature aging are issues, since it is assumed that the characteristics are not changed and the microstructure is not changed after room temperature aging for approximately 100 days, it is more preferable that the microstructure and characteristics of a sheet after room temperature aging has sufficiently advanced, that is, after the above-described series of refining have been carried out, and then 100 or more days have passed are examined and evaluated.

Definition of Cluster of the Present Embodiment

The microstructure in an arbitrary center part in the thickness direction of such an aluminum alloy sheet is measured by a three-dimensional atom probe field ion microscope. As the cluster present in the measured microstructure, according to the present embodiment, first, the cluster contains either or both of Mg atoms and Si atoms by a total of 10 pieces or more. Further, the number of pieces of the Mg atoms or the Si atoms contained in the aggregate of atoms is preferably as large as possible and the upper limit thereof is not particularly limited. However, from the manufacturing limit, the upper limit of the number of pieces of Mg atoms and Si atoms contained in the cluster is approximately 10000 pieces.

In Patent Document 2 described above, the cluster contains either or both of Mg atoms and Si atoms by a total of 30 pieces or more. However, in the present embodiment, as described above, since clusters with comparatively small size inhibit BH responses, they are regulated so that the number thereof becomes small. For this reason, in order to control the clusters with comparatively small size which need to be regulated to be in a measurable range, it is stipulated that either or both of Mg atoms and Si atoms are contained in a total of 10 pieces or more, similar to Patent Document 3 described above.

Further in the present embodiment, similar to Patent Documents 2 and 3 described above, one in which, when any atom of the Mg atoms and the Si atoms contained in the cluster is used as a reference, the distance between the atom as the reference and any atom among other atoms adjacent thereto is 0.75 inn or less is set as an aggregate (cluster) of atoms stipulated in the present embodiment (satisfying the stipulation of the present embodiment). The distance therebetween of 0.75 nm is a numerical value determined in order to assure the number density of the cluster with a large size in which the distance between atoms such as Mg and Si is short and which is effective in improving BH responses after long-term room temperature aging, and to regulate the clusters with a small size to control the number density thereof to be small. The present inventors have hitherto performed an intensive research on the relationship between the aluminum alloy sheet capable of exerting excellent BH responses even in the vehicle body bake treatment and the aggregate of an atomic level. As a result, it has been found through experimentation that the high number density of the aggregates of atoms stipulated by the definition described above represents the form of the microstructure exerting good BH responses. Therefore, although the technical implication of the distance of 0.75 nm between atoms has not been sufficiently clarified, it is important for the purpose of strictly assuring the number density of an aggregate of atoms exerting good BH responses and is a numerical value determined for the purpose.

The cluster stipulated in the present embodiment contains both of Mg atoms and Si atoms in most cases. However, the cases where Mg atoms are contained but Si atoms are not contained, or the cases where Si atoms are contained but Mg atoms are not contained are involved. Further, it is not constantly configured only of the Mg atoms and the Si atoms, and there is a high probability that Al atoms are additionally contained.

Moreover, depending on the component composition of the Al—Mg—Si alloy sheet containing Sn which is the object of the present embodiment, a case inevitably exists, in which atoms such as Sn, Fe, Mn, Cu, Cr, Zr, V, Ti, Zn, or Ag contained as alloy elements or impurities, are contained in the cluster and these other atoms are counted by 3DAP analysis. However, even when these other atoms (derived from alloy elements or impurities) are contained in the cluster, these are lower levels than the total number of Mg atoms and Si atoms. Therefore, even in a case where such other atoms are contained in the cluster, those satisfying the stipulation (condition) function as the cluster of the present embodiment in the same manner as in the cluster formed of only Mg atoms and Si atoms. Accordingly, the cluster stipulated in the present embodiment may contain any other atoms, as long as the stipulation described above is satisfied.

In addition, the expression “when any atom of Mg atoms and Si atoms contained therein is used as a reference, the distance between the atom as the reference and any atom among other atoms adjacent thereto is 0.75 mm or less” of the present embodiment means that all of the Mg atoms and the Si atoms present in the cluster contain, in the periphery thereof, at least one Mg atom or Si atom with the distance therebetween being 0.75 nm or less.

With respect to the stipulation on the distance between atoms in the cluster of the present embodiment, when any atom of Mg atoms and Si atoms contained therein is used as a reference, all distances between the atom as the reference and all atoms of other atoms adjacent thereto are not necessarily 0.75 nm or less. On the contrary, all of them may be 0.75 nm or less. In other words, other Mg atoms or Si atoms whose distance therebetween exceeds 0.75 nm may be adjacent to each other, and at least one of other Mg atoms and Si atoms satisfying the stipulated distance (space) may be present in the periphery of a specific (serving as a reference) Mg atom or Si atom.

In a case where one other Mg atom or Si atom adjacent to the reference atom, which satisfies the stipulated distance is present, the number of Mg atoms or Si atoms to be counted, that satisfy the condition of the distance is 2 including the specific (serving as a reference) Mg atom or Si atom. In the case where two other Mg atoms or Si atoms adjacent to the reference atom, which satisfy the stipulated distance is present, the number of Mg atoms or Si atoms to be counted, that satisfy the condition of the distance is 3 including the specific (serving as a reference) Mg atom or Si atom.

The cluster described above is a cluster generated by a temperature holding treatment after a solution heat treatment and a stopping of a quenching treatment at a high temperature in refining after rolling described above and below in detail. That is, the cluster of the present embodiment is an aggregate of atoms generated by a temperature holding treatment after a solution heat treatment and a stopping of a quenching treatment at a high temperature, and is a cluster which contains either or both of Mg atoms and Si atoms by a total of 10 pieces or more and, when any atom of Mg atoms and Si atoms contained therein is used as a reference, the distance between the atom as the reference and any atom among other atoms adjacent thereto is 0.75 nm or less.

Until now, it has been reported that a cluster promoting precipitation of a GP zone or a β′ phase that increases the strength in artificial aging or a bake treatment is a Mg/Si cluster as described above, and this cluster is formed by a heat treatment at 50° C. to 150° C. after the solution heat treatment and the quenching treatment. Meanwhile, a cluster that suppresses precipitation of a GP zone or a β′ phase in the artificial aging treatment or the bake treatment is an Si-rich cluster and the cluster is formed by retention at room temperature (room temperature aging) after the solution heat treatment and the quenching treatment (for example, described in Sato, Journal of The Japan Institute of Light Metals, vol. 56, p. 595).

However, in a producing process of a typical aluminum alloy, since a sheet is left at room temperature typically for 1 month to 4 months (stands at room temperature) after the production thereof until the forming work into the panel is performed by an automobile manufacturer as described above, a microstructure is inevitably in the form in which a Mg—Si cluster generated during production of the sheet and a cluster rich in Si generated during room temperature aging coexist and thus it is difficult to generate only a Mg—Si cluster promoting the BH responses.

Here, since the present inventors considered that it is important to control the proportions of the clusters rich in Si which adversely affect the BH responses and the Mg—Si clusters which promote the BH responses in order to improve the BH responses, they evaluated the number density of the clusters and the components thereof in detail and clarified the form of the cluster that improves the BH responses.

Stipulation of Composition of Cluster According to Present Embodiment

Even in a case of clusters defined in the present embodiment or clusters satisfying the preconditions have difference influences on the BH responses depending on the compositions thereof. Clusters rich in Si atoms adversely affect the BH responses. This is because the clusters rich in Si are generated during the bake treatment and a difference in a Mg/Si composition with a strengthening phase such as β″ or β′ that improves the BH responses is comparatively large and thus generation of strengthening phases during the bake treatment is not promoted but the generation of strengthening phases is inhibited.

Meanwhile, clusters rich in Mg atoms improve the BH responses. This is because the clusters rich in Mg are generated during the bake treatment and a Mg/Si composition is comparatively similar to that in a strengthening phase such as β″ or β′ that improves the BH responses and thus generation of strengthening phases during the bake treatment is promoted.

In the present embodiment, among these clusters, the proportion of the clusters having a large number of Mg atoms is controlled to be high based on such the relationship in composition of clusters to improve the BH responses. For this reason, in the present embodiment, among the aggregates satisfying conditions in which either or both of an Mg atom and an Si atom are contained by a total of 10 pieces or more and, when any atom of the Mg atom and the Si atom contained therein is used as a reference, a distance between the atom as the reference and any atom among other atoms adjacent thereto is 0.75 nm or less, the proportion of aggregates of atoms rich in Mg atoms in which the ratio (Mg/Si) of the number of Mg atoms to the number of Si atoms is ½ or greater is stipulated as 0.70 or greater. When the proportion of the aggregates of atoms in which the Mg/Si ratio is ½ or greater is less than 0.70, the number of clusters rich in Si atoms becomes great and thus the BH responses are likely to be degraded due to the above-described mechanism.

Here, the upper limit of the proportion of the aggregates of atoms in which the Mg/Si ratio is ½ or greater is not particularly limited, but approximately 0.95 is the limit in manufacturing.

(Density of Cluster)

The clusters defined or the clusters satisfying the preconditions as described above are controlled to be contained at an average number density of 3.0×10²³ pieces/m³ or greater and 25.0×10²³ pieces/m³ or less in the present embodiment. When the average number density of the clusters stipulated in the present embodiment is smaller than 3.0×10²³ pieces/m³, the formation amount of the clusters themselves becomes insufficient. This means that plenty of added (contained) Mg and Si are consumed by the clusters formed by the room temperature aging described above and degradation of the BH responses and deterioration of workability are thus caused after standing at room temperature (room temperature aging).

On the other hand, the upper limit of the average number density of the clusters is approximately 25.0×10²³ pieces/m³ (approximately 2.5×10²⁴ pieces/m³) which is stipulated from the limit in manufacturing.

(Measurement Principle and Measurement Method by 3DAP)

The measurement principle and the measurement method by a 3DAP of the present invention are disclosed in Patent Documents 2 and 3. That is, the 3DAP (three-dimensional atom probe) is a field ion microscope (FIM) attached with a time of flight mass spectrometer. With such the constitution, it is a local analyzer capable of observing respective atoms on the metal surface by the field ion microscope and identifying these atoms by time of flight mass spectrometry. Also, the 3DAP is a means very effective for structural analysis of the aggregates of atoms because it can simultaneously analyze the kind and position of atoms emitted from the sample. Therefore, as described above, it is used for analysis of the microstructure of a magnetic recording film, an electronic device, steel material or the like as a widely known technique. Further, recently, it is also used for determination and the like of the cluster of the microstructure of an aluminum alloy sheet as described above.

The 3DAP utilizes an ionizing phenomenon of sample atoms themselves under a high electric field which is called electric field evaporation. When high voltage required by the sample atoms for electric field evaporation is applied to the sample, atoms are ionized from the sample surface, pass through a probe hole, and reach a detector.

This detector is a position sensitive detector, which carries out mass spectroscopy of respective ions (identification of elements that are atomic species), measures the time of flight until each ion reaches the detector, and can thereby simultaneously determine the detected position (atomic structural position). Accordingly, the 3DAP can simultaneously measure the position and the atomic species of the atom at the tip of the sample, and therefore has the feature of being able to three-dimensionally reconstitute and observe the atomic structure of the tip of the sample. Also, because electric field evaporation takes place in order from the tip surface of the sample, distribution in the depth direction of the atoms from the tip of the sample can be examined with the resolution of an atomic level.

Because the 3DAP utilizes a high electric field, the sample to be analyzed should have high electro-conductivity of metal and the like, and the shape of the sample is generally required to be ultra fine needle shape with the tip diameter of approximately 100 nm diameter or less than that. Therefore, a sample is taken from the center part in the sheet thickness direction and the like of an aluminum alloy sheet that becomes an object to be measured, the sample is cut and electropolished by a precise cutting apparatus, and a sample having an ultra fine needle shape tip section for analysis is manufactured. As a measuring method, “LEAP 3000” made by Imago Scientific Instruments Corporation is used for example, high pulse voltage of 1 kV order is applied to the aluminum alloy sheet sample whose tip is formed into a needle shape, and several million pieces of atoms are continuously ionized from the tip of the sample. The ion is detected by the position sensitive type detector and is applied with pulse voltage. Mass spectroscopy of the ion (identification of the element that is the atomic species) is carried out based on the time of flight after the respective ions come out from the tip of the sample until reaching the detector.

Also, utilizing the characteristic that the electric field evaporation takes place regularly in order from the tip surface of the sample, a coordinate in the depth direction is properly given to a two-dimensional map that shows the arrival location of the ion, and three-dimensional mapping (construction of atomic structure: atom map in three dimensions) is executed by using an analytical software “IVAS”. Thus, a three-dimensional atom map of the tip of the sample can be obtained.

With respect to this three-dimensional atom map, the aggregate of atoms (cluster) is further analyzed by using a Maximum Separation Method that is a method for defining an atom belonging to a precipitate and a cluster. In this analysis, the number of either or both of Mg atoms and Si atoms (10 pieces or more in total), the distance (space) between the Mg atom or the Si atom neighboring each other, and the number of either of the Mg atom or the Si atom having the predetermined narrow space (0.75 nm or less) are given as parameters.

Also, in the first embodiment of the present invention, either or both of an Mg atom and an Si atom are contained by a total of 10 pieces or more and, when any atom of the Mg atom and the Si atom contained therein is used as a reference, a distance between the atom as the reference and any atom among other atoms adjacent thereto is 0.75 mu or less, and the cluster satisfying these conditions is defined to be the aggregate of atoms of the present embodiment. Then, the dispersion state of the aggregates of atoms matching this definition is evaluated, and the number density of the aggregates of atoms is measured and quantified as the average density per 1 m³ (pieces/m³) by averaging for 3 or more measurement samples.

That is, a maximum rotational radius I_(g) when the aggregate of atoms as the measurement object is regarded as a sphere using the analysis software originally specific to the 3DAP is acquired by using the following formula of Math. 1.

$\begin{matrix} {l_{g} = \sqrt{\frac{\sum\limits_{i = 1}^{n}\; \left\lbrack {\left( {x_{i} - \overset{\_}{x}} \right)^{2} + \left( {y_{i} - \overset{\_}{y}} \right)^{2} + \left( {z_{i} - \overset{\_}{z}} \right)^{2}} \right\rbrack}{n}}} & \left\lbrack {{Math}.\mspace{14mu} 1} \right\rbrack \end{matrix}$

In the formula of Math. 1, I_(g) represents a rotational radius automatically calculated by software specific to a three-dimensional atom probe field ion microscope. x, y, and z respectively represent an x axis, a y axis, and a z axis which are invariable in the measuring layout of the three-dimensional atom probe field ion microscope and x_(i), y_(i), and z_(i) respectively represent the lengths of the x axis, the y axis, and the z axis and spatial coordinates of Mg atoms and Si atoms constituting the aggregate of atoms. “x bar” and the like in which “-” is placed on the top of “x,” “y,” or “z” also represent the lengths of the x axis, the y axis, and the z axis, but are barycentric coordinates of the aggregates of atoms. n represents the number of Mg atoms and Si atoms constituting the aggregates of atoms.

Next, the rotational radius I_(g) is converted into the Guinier radius r_(G) by using the relationship of the following formula r_(G)=√(5/3)·I_(g) of Math. 2.

$\begin{matrix} {r_{G} = {\sqrt{\frac{5}{3}} \cdot l_{g}}} & \left\lbrack {{Math}.\mspace{14mu} 2} \right\rbrack \end{matrix}$

The converted Guinier radius r_(G) is regarded as the radius of the aggregate of atoms and a circle equivalent diameter r which is the maximum value of the aggregates of atoms as the measurement object is calculated. Further, the number n of the aggregates of atoms satisfying the preconditions is also calculated. Further, the average number density (pieces/m³) of the aggregates of atoms satisfying the preconditions can also be calculated from the number n.

Measurements of the clusters with the 3DAP are performed on 10 sites of an arbitrary center part of the Al—Mg—Si alloy sheet after being subjected to the refining, and the measurement values (calculated values) are averaged to set an average value stipulated in the present embodiment.

Further, the average radius E(r) (nm) of the circle equivalent diameters of the aggregates of atoms is calculated from the circle equivalent radius r which is the calculated maximum value and the number n of the aggregates of atoms satisfying the preconditions by using the formula “E(r)=(1/n)Σr” described above.

In addition, the standard deviation a of the circle equivalent diameters of the aggregates of atoms satisfying the preconditions described above is acquired from the average radius E(r) of the circle equivalent diameters by using the formula “σ²=(1/n)Σ[r−E(r)]²” described above.

The calculation formula of the radius of the aggregate of atoms, and measurement and the conversion method of from the rotational radius I_(g) to the Guinier radius r_(G) use M. K. Miller: Atom Probe Tomography, (Kluwer Academic/Plenum Publishers, New York, 2000), p. 184 as a reference. In addition thereto, the calculation formulae of the radius of aggregates of atoms are described in many literatures. For example, “(2) Three-dimensional atom probe analysis” on the page 140 of “Microstructural evolution in low alloy steels under high dose ion irradiation” (Katsuhiko Fujii, Koji Fukuya, Tadakatsu Ohkubo, Kazuhiro Hono, et al.) describes the formula of Math. 1 described above and the conversion formula to the Guinier radius r_(G) (in this case, the symbol of the rotational radius I_(g) is described as r_(G)).

Moreover, in the second embodiment of the present invention, either or both of an Mg atom and an Si atom are contained by a total of 10 pieces or more and, when any atom of the Mg atom and the Si atom contained therein is used as a reference, a distance between the atom as the reference and any atom among other atoms adjacent thereto is 0.75 nm or less, and a cluster satisfying these conditions is claimed to be the aggregate of atoms of the present embodiment. Then, the dispersion state of the aggregates of atoms matching this definition is evaluated, and the number density of the aggregates of atoms is measured and quantified as the average density per 1 m³ (pieces/m³) by averaging for 3 or more measurement samples.

In addition, the number N_(cluster) of Mg atoms and Si atoms contained in all aggregates of atoms satisfying the conditions is acquired. Further, the number N_(total) of all Mg atoms and Si atoms which are detected by a detector and are contained in both of a solid solution and aggregates of atoms, that is, measured by the 3DAP is acquired. Further, the ratio of N_(cluster) to N_(total) is acquired from the formula “N_(cluster)/N_(total)×100” and to the average value (average ratio) is controlled to be 1% or more and 15% or less.

Furthermore, a maximum rotational radius I_(g) when the aggregate of atoms as the measurement object is regarded as a sphere using the analysis software originally specific to the 3DAP is acquired by using the above formula of Math. 1.

Next, the rotational radius I_(g) is converted into the Guinier radius r_(G) by using the relationship of the above formula of Math. 2, r_(G)=√(5/3)·I_(g).

The converted Guinier radius r_(G) is regarded as the radius of the aggregate of atoms and a circle equivalent diameter r which is the maximum value of the aggregates of atoms as the measurement object is calculated. Further, the number n of the aggregates of atoms satisfying the preconditions is also calculated. Further, the average number density (pieces/m³) of the aggregates of atoms satisfying the preconditions can also be calculated from the number n.

Measurements of the clusters with the 3DAP are performed on 10 sites of an arbitrary center part of the Al—Mg—Si alloy sheet after being subjected to the refining, and the measurement values (calculated values) are averaged to set an average value stipulated in the present embodiment.

Further, the average radius E(r) (nm) of the circle equivalent diameters of the aggregates of atoms is calculated from the circle equivalent radius r which is the calculated maximum value and the number n of the aggregates of atoms satisfying the preconditions by using the formula “E(r)=(1/n)Σr” described above.

Moreover, in the third embodiment of the present invention, either or both of an Mg atom and an Si atom are contained by a total of 10 pieces or more and, when any atom of the Mg atom and the Si atom contained therein is used as a reference, a distance between the atom as the reference and any atom among other atoms adjacent thereto is 0.75 nm or less, and a cluster satisfying these conditions is defined to be the aggregate of atoms of the present embodiment. Then, the dispersion state of the aggregates of atoms matching this definition is evaluated, and the number density of the aggregates of atoms is measured and quantified as the average density per 1 m³ (pieces/m³) by averaging for 3 or more measurement samples.

(Detection Efficiency of Atoms by Using 3DAP)

Currently, the detection efficiency of atoms by using the 3DAP is limited to approximately 50% of ionized atoms and the remaining atoms cannot be detected. When the detection efficiency of atoms by using the 3DAP is improved or the like in the future, that is, varies greatly, there is a possibility that the measurement result by the 3DAP of the average number density (piece/μm³) of clusters having each size stipulated in the present invention may vary. Therefore, in order to provide reproducibility for this measurement, it is preferable that the detection efficiency of atoms by using the 3DAP is set to approximately 50%, which is a substantially predetermined value.

(Chemical Component Composition)

Next, the chemical component composition of the 6000 series aluminum alloy sheet will be described below. With respect to the 6000 series aluminum alloy sheet that is the object of the present invention, various properties such as excellent formability, BH responses, strength, weldability, and corrosion resistance are required as a sheet or the like for an outer sheet of an automobile described above.

In order to satisfy such requirements, the composition of the aluminum alloy sheet contains, in mass %, Mg: 0.2% to 2.0%, Si: 0.3% to 2.0% and Sn: 0.005% to 0.3%, with the remainder being Al and inevitable impurities. Further, all of the % indications of the content of each element mean mass %. Moreover, in the present specification, the percentage (mass %) based on the mass is the same as the percentage (weight %) based on the weight. In addition, “X % or less (not including 0%)” of the content of each chemical component is sometimes expressed by “more than 0% and X % or less.”

The 6000 series aluminum alloy sheet which is the object of the present invention is preferable to be such a 6000 series aluminum alloy sheet with excess Si with excellent BH responses in which the mass ratio Si/Mg of Si to Mg is 1 or greater. The 6000 series aluminum alloy sheet secures the formability by lowering the proof stress during press forming and bending, and has excellent age-hardenability (BH responses) with which the proof stress increases by age hardening through heating during the artificial aging treatment at a comparatively low temperature such as the bake treatment of a panel after forming, and required strength can be secured. Among them, the 6000 series aluminum alloy sheet with excess Si is excellent in the BH responses in comparison to the 6000 series aluminum alloy sheet in which the mass ratio Si/Mg is less than 1.

According to the present invention, elements other than these Mg and Si are basically impurities or elements that may be contained, and are set to have the content (allowable amount) of each element level in conformity with the AA or JIS standard or the like.

Even in the present invention, in a case of using, by a large amount, not only the high purity Al matrix but also the 6000 series alloy, other aluminum alloy scrap materials, a low purity Al matrix, and the like containing elements other than Mg and Si by a large amount as additive elements (alloy elements) as the melting raw material of an alloy from the viewpoint of resources recycling, other elements described below are inevitably mixed in by a substantial amount. Moreover, refining itself that daringly reduces these elements involves cost increase, and inclusion of them to some extent should be allowed. Further, even when a substantial amount may be contained, there is an inclusion range not impeding the object and effects of the present invention.

Accordingly, in the present invention, inclusion of such elements described below is allowed in a range of equal to or less than an upper limit in conformity with the AA or JIS standard or the like stipulated as described below. Specifically, one kind or two or more kinds among Mn: 1.0% or less (not including 0%), Cu: 1.0% or less (not including 0%), Fe: 1.0% or less (not including 0%), Cr: 0.3% or less (not including 0%), Zr: 0.3% or less (not including 0%), V: 0.3% or less (not including 0%), Ti: 0.1% or less and preferably 0.05% or less (not including 0%), Zn: 1.0% or less (not including 0%), and Ag: 0.2% or less (not including 0%) may be further contained in these ranges in addition to the fundamental composition described above. In a case where these elements are contained, since the corrosion resistance is likely to be degraded when the content of Cu is great, the content of Cu is preferably 0.7% or less and more preferably 0.3% or less. Further, when the contents of Mn, Fe, Cr, Zr, and V are great, comparatively coarse compounds are likely to be generated and the hem bendability is thus likely to be degraded. For this reason, the content of Mn is preferably 0.6% or less and more preferably 0.3% or less and the each content of Cr, Zr, and V is preferably 0.2% or less and more preferably 0.1% or less. The inclusion range and the importance, or the allowable amount of each element in the above-described 6000 series aluminum alloy will be described below.

Si: 0.3% to 2.0%

Along with Mg, Si is an important element in forming the cluster stipulated in the present invention. Further, it is an indispensable element that forms aging precipitates contributing to strengthening of solid solution and improvement of the strength during the artificial aging treatment at a low temperature described above such as the bake treatment to exert age-hardenability, thereby obtaining the strength (proof stress) required as an outer panel of an automobile. Further, it is the most important element for providing the 6000 series aluminum alloy sheet of the present invention with a combination of various characteristics such as the total elongation affecting the press formability. Moreover, in order to exert excellent age-hardenability in the bake treatment at a lower temperature for a shorter period of time after forming into a panel, preferred is a 6000 series aluminum alloy composition in which the Si/Mg in mass ratio is 1.0 or greater and which further contains Si more excessively high relative to Mg than the generally referred to as an excessive-Si type.

When the content of Si is excessively low, since the absolute amount of Si is insufficient, the cluster stipulated in the present invention cannot be formed by the stipulated number density, and the bake hardenability extremely deteriorates. Further, various properties such as the total elongation required for respective uses cannot be achieved simultaneously. Meanwhile, when the content of Si is excessively high, coarse constituents and precipitates are formed, and bendability, total elongation and the like extremely deteriorate. Further, the weldability is also extremely impeded. Therefore, Si is in a range of 0.3% to 2.0%. The lower limit thereof is more preferably 0.6% and the upper limit thereof is more preferably 1.4%.

Mg: 0.2% to 2.0%

Along with Si, Mg is also an important element in forming the cluster stipulated in the present invention. Further, it is an indispensable element that forms aging precipitates contributing to strengthening of solid solution and improvement of the strength along with Si during the artificial aging treatment such as the bake treatment to exert age-hardenability, thereby obtaining a required proof stress as a panel.

When the content of Mg is excessively low, since the absolute amount of Mg is insufficient, the cluster stipulated in the present invention cannot be formed by the stipulated number density, and the bake hardenability extremely deteriorates. Therefore, the proof stress required as a panel cannot be obtained. Meanwhile, when the content of Mg is excessively high, coarse constituents and precipitates are formed, and bendability, total elongation and the like extremely deteriorate. Therefore, the content of Mg is in a range of 0.2% to 2.0%. The lower limit thereof is more preferably 0.3% and the upper limit thereof is more preferably 1.0%. Moreover, it is preferable to be such an amount that the Si/Mg in mass ratio is 1.0 or greater.

Sn: 0.005% to 0.3%

Sn traps vacancies at room temperature and thereby suppresses diffusion at room temperature and suppresses generation of clusters at room temperature. For this reason, Sn has an effect of reducing As proof stress in both of the initial period (7 days) of room temperature aging and the late period (100 days) of room temperature aging and improving the hem workability. Since the trapped vacancies are released at the high temperature during being subjected to the baking finish, the diffusion is promoted in contrast and the BH responses can be improved. For this reason, even in a case of clusters having the comparable number density, BH responses can be improved in a case where Sn is contained compared to a case where Sn is not contained. When the content of Sn is excessively small, the generation of clusters at room temperature cannot be suppressed and the number density of the clusters becomes excessively greater or the average ratio (N_(cluster)/N_(total))×100 of N_(cluster) to N_(total) as described above exceeds 15% in some cases. For this reason, the As proof stress is extremely large after retention at room temperature for 100 days so that the press formability or the hem workability is degraded, the number of strengthening phases (β″) generated during the artificial aging hardening treatment is decreased, and the BH responses are likely to be degraded. Therefore, the content of Sn is in a range of 0.005% to 0.3%. The lower limit thereof is more preferably 0.01% and the upper limit thereof is more preferably 0.2%. The microstructure of the Al—Si—Mg alloy sheet containing Sn is also different from one which does not contain Sn, as described below. In this case, even when Sn is contained in the same manner, since the microstructures thereof may vary if the production conditions are different from each other, a microstructure of the present invention, which is effective in suppressing room temperature aging at a high level and improving the bake hardening is not necessarily obtained.

(Manufacturing Method)

Next, a manufacturing method of the aluminum alloy sheet of the present invention will be described below. With respect to the aluminum alloy sheet of the present invention, the manufacturing process itself is an ordinary method or widely known method, and it is manufactured by casting an aluminum alloy slab having the above-described 6000 series component composition, performing a homogenizing heat treatment, subjecting to hot rolling and cold rolling to obtain a predetermined sheet thickness, and further subjecting to a refining treatment such as a solution heat treatment and a quenching treatment.

However, in order to control the cluster of the present invention for improving the BH responses during these manufacturing processes, the solution heat treatment, the quenching treatment, the suitable quenching (cooling) stop temperature, and retention in the temperature range thereof are required to be controlled more appropriately as described below. Further, in other processes, there are also preferable conditions for controlling the cluster within the stipulated range of the present invention.

(Melting and Casting Cooling Rate)

First, in the melting and casting process, the aluminum alloy molten metal that has been molten so as to be adjusted within the 6000 series component composition range is casted by properly selecting a typical melting and casting method such as a continuous casting method or a semi-continuous casting method (DC casting method). Here, in order to control the cluster within the stipulated range of the present invention, it is preferable to make the average cooling rate in casting as high (quick) as possible at 30° C./min or greater from the liquidus temperature to the solidus temperature.

In a case where such temperature (cooling rate) control in a high temperature range during casting is not performed, the cooling rate in this high temperature range inevitably becomes slow. In a case where the average cooling rate in the high temperature range becomes slow, the amount of the constituents formed coarse in the temperature range of this high temperature region increases, and the unevenness of the size or the amount of the constituents in the sheet thickness direction and the width direction of the slab increases. As a result, the possibility that the stipulated cluster cannot be controlled to the range of the present invention increases.

(Homogenizing Heat Treatment)

Next, the casted aluminum alloy slab is subjected to the homogenizing heat treatment prior to hot rolling. The object of the homogenizing heat treatment (soaking treatment) is homogenization of the microstructure, that is, to eliminate segregation within the grains in the microstructure of the slab. The treatment is not particularly limited as long as it is in the condition for achieving the object, and can be the treatment typically carried out once or one step.

The homogenizing heat treatment temperature is suitably selected from a range of 500° C. or higher and lower than the melting point, and the homogenizing time is suitably selected from 4 hours or longer. When this homogenizing temperature is low, the segregation within the grains cannot be eliminated sufficiently, which acts as the fracture origin, and therefore the stretch flangeability and bendability deteriorate. Even when hot rolling is started immediately thereafter or hot rolling is started after cooling and being retained to a suitable temperature, it is possible to control the number density of the cluster to that stipulated in the present invention.

After the homogenizing heat treatment is performed, it is possible to perform cooling to room temperature with an average cooling rate of 20° C./h to 100° C./h between 300° C. and 500° C., and performing reheating to 350° C. to 450° C. with an average cooling rate of 20° C./h to 100° C./h, and then to start hot rolling in this temperature range.

When the conditions of the average cooling rate after the homogenizing heat treatment and the reheating rate thereafter are deviated from, the possibility of formation of the coarse Mg—Si compounds increases.

(Hot Rolling)

Hot rolling is constituted of a rough rolling process of a slab and a finish rolling process according to the thickness of the sheet to be rolled. In these rough rolling process and finish rolling process, rolling mills such as a reverse type or a tandem type are suitably used.

Here, since burning occurs under the condition in which the hot rolling (rough rolling) start temperature exceeds the solidus temperature, hot rolling itself is difficult to carry out. Further, when the hot rolling start temperature is lower than 350° C., a load during the hot rolling becomes exceedingly great and thus the hot rolling itself is difficult to carry out. Therefore, the hot rolling start temperature is in a range of 350° C. to the solidus temperature and more preferably in a range of 400° C. to the solidus temperature.

(Annealing of Hot Rolled Sheet)

Annealing before cold rolling (rough annealing) of the hot rolled sheet is not necessarily required, but may be performed in order to further improve the characteristics such as formability by miniaturizing grains and optimizing the texture.

(Cold Rolling)

In cold rolling, the above-described hot rolled sheet is rolled and manufactured into a cold rolled sheet (including a coil) of a desired final sheet thickness. However, in order to further miniaturize the grains, the cold rolling ratio is preferably 60% or greater, and intermediate annealing may be performed between cold rolling passes with an object similar to that of the rough annealing described above.

(Solution Heat Treatment and Quenching Treatment)

After the cold rolling, the solution heat treatment and the quenching treatment are performed. The solution heat treatment and the quenching treatment can be performed by heating and cooling by using a typical continuous heat treatment line and are not particularly limited. However, because it is preferable to obtain a sufficient solid solution amount of each element and that the grain size is finer as described above, it is preferable that the treatments are performed under the conditions in which heating is carried out at a solution heat treatment temperature of 520° C. or more, and equal to or less than the melting temperature, at a heating rate of 5° C./sec or greater, and then retention is carried out for 0 second to 10 seconds.

In addition, from the viewpoint of suppressing formation of coarse intergranular compounds that deteriorate the formability and hem workability, it is preferable that the average cooling rate from the solution heat temperature to the quenching stop temperature is 3° C./s or greater. When the cooling rate of the solution heat treatment is low, coarse Mg₂Si and an element Si are generated during cooling and thus formability is degraded. Moreover, the amount of solid solution after the solution heat treatment is reduced and then the BH responses are degraded. In order to secure this cooling rate, for the quenching treatment, means and conditions for air cooling by a fan or the like, or water cooling by mist, spray, immersion or the like are selectively used.

(Temperature Retaining Treatment after Quenching Stop)

Here, in the first embodiment of the present invention, the quenching treatment is performed, not by cooling a sheet after the solution heat treatment to room temperature but by carrying out the retention treatment in which cooling (quenching) is stopped in a temperature range T where the sheet is 80° C. to 130° C. and the sheet is retained in this temperature range for a predetermined time t and then it is cooled to room temperature by using any kind of cooling means such as standing to cool or forced cooling. The retention (treatment) in a temperature of 80° C. to 130° C. may be performed by heating or non-heating and may be isothermal retention or may have a temperature gradient. The retention time t at this temperature is determined to satisfy the following formula in relation to the quenching stop temperature T.

1.6×10⁴×exp[−0.096×T]<t<4.3×10⁵×exp[−0.097×T]

In the present embodiment, as to the conditions for stopping the quenching treatment for the purpose of obtaining size distribution of stipulated predetermined clusters, the relationship among various quenching stop temperatures, retention times and the like has been examined in detail. As a result, the size distribution of clusters during temperature retention after the quenching stop is greatly affected by the diffusion distance of Mg or Si. In the analyzed results, it is preferable that the diffusion distance of Mg or Si is set to a range of 1.3×10⁻⁹ m to 6.5×10⁻⁹ m.

The diffusion distance of Mg or Si is expressed by the following formula of Math. 3. In the formula, D₀ represents a diffusion coefficient represented by formula of Math. 4 and the value is 6.2×10⁻⁶ (m²/s). Q represents an activation energy for diffusion and the value is 11500 (J/mol). R represents a gas constant and the value is 8.314.

The quenching stop temperature for obtaining a preferred range of the diffusion distance of Mg or Si and the temperature retention time after the quenching stop are re-organized, and the upper limit and the lower limit of the temperature retention time t after the quenching stop are determined as shown in the above-described formula in relation to the quenching stop temperature T.

$\begin{matrix} \sqrt{D_{0}{\exp \left\lbrack {- \frac{Q}{RT}} \right\rbrack}t} & \left\lbrack {{Math}.\mspace{14mu} 3} \right\rbrack \\ \frac{J}{{mol} \cdot K} & \left\lbrack {{Math}.\mspace{14mu} 4} \right\rbrack \end{matrix}$

The size distribution of the predetermined clusters stipulated in the present embodiment is obtained by the cooling stop and the temperature retaining treatment at a comparatively high temperature in the quenching treatment. Further, the average number density of the aggregates of atoms stipulated in the present embodiment which is 2.5×10²³ pieces/m³ or greater and 20.0×10²³ pieces/m³ or less. This is because most of the aggregates of atoms stipulated in the present embodiment are formed such that the size thereof is equal or similar to each other during the processes of cooling stop and the temperature retaining treatment at a comparatively high temperature in the quenching treatment.

In other words, most of the aggregates of atoms which contain either or both of an Mg atom and an Si atom by a total of 10 pieces or more and in which, when any atom of the Mg atom and the Si atom contained therein is used as a reference, the distance between the atom as the reference and any atom among other atoms adjacent thereto is 0.75 nm or less are formed during the processes of the cooling stop and the temperature retaining treatment at a comparatively high temperature in the quenching treatment. Also, since the size of most of the aggregates of atoms which are completely formed during the temperature retaining treatment is equal or similar to each other, the uniformity in size can be obtained that satisfies the conditions in which the average radius of the circle equivalent diameters is 1.15 nm or greater and 1.45 nm or less and the standard deviation of radii of the circle equivalent diameters is 0.45 nm or less.

Consequently, in the sheet which is subjected to the cooling stop and the temperature retaining treatment at a comparatively high temperature in the quenching treatment, the number of clusters formed by room temperature aging at room temperature after the temperature retaining treatment is reduced and the room temperature aging becomes lower. For this reason, press formabilities including bendability are improved and, even after long-term retention at room temperature, by performing heating during the artificial aging treatment at 170° C. for 20 minutes such as the bake treatment on the panel thereafter, excellent BH responses in which a difference in 0.2% proof stress before and after BH is 90 MPa or greater can be obtained.

Even in a case where the quenching to room temperature is performed and then the re-heating treatment (annealing treatment) is performed without stopping the quenching at such a high temperature as in the related art or a case where the cooling stop temperature during quenching is room temperature, which is extremely low, a sheet itself can be manufactured. In this case, even when clusters whose size is equal or similar to each other, stipulated in the present embodiment, are formed, there is a possibility that the absolute number thereof is not small or the standard deviation becomes greater. Accordingly, a disadvantage that the stipulation of the cluster of the present embodiment cannot be satisfied with high reproducibility is generated.

(Cooling after Temperature Retaining Treatment)

The cooling to room temperature after the temperature retaining treatment may be standing to cool or forced rapid cooling by using the cooling means as in the quenching for efficiency in production. That is, since the clusters whose size is equal or similar to each other, stipulated in the present embodiment, completely come out through the temperature retaining treatment, forced rapid cooling as in a conventional re-heating treatment or controlling the average cooling rate which is performed in a complicated manner over several stages is not necessary.

(Re-Heating Treatment)

Further, in the second embodiment of the present invention, the re-heating treatment is performed after the solution heat treatment and the quenching treatment. The re-heating treatment is carried out in two stages, and the first stage of the treatment is carried out in a temperature range of 80° C. to 250° C. which is the attaining temperature (heating temperature) for a retention time in a range of several seconds to several minutes. The first stage of cooling after the re-heating treatment may be standing to cool or forced rapid cooling by using the cooling means of the solution heat treatment and the quenching treatment for efficiency in production. Then, when the cooling is finished after the first stage of re-heating treatment is carried out, within 24 hrs of the retention time at room temperature, the second stage of the re-heating treatment is carried out in a temperature range of 70° C. to 130° C. which is the attaining temperature (heating temperature) for a retention time in a range of 3 hrs to 48 hrs. If the retention time at room temperature when the cooling after the first stage of re-heating treatment is finished exceeds 24 hrs, the room temperature aging excessively proceeds and thus the effects of the second stage of re-heating treatment is reduced.

In a case where the conditions are deviated from such re-heating treatment conditions, the average content obtained by summing Mg and Si contained in the aggregates of atoms is difficult to be set to 10% or greater and 30% or less of the content obtained by summing Mg and Si contained in the aluminum alloy sheet. For example, when the attaining temperature of the first stage of re-heating is lower than 100° C. or the attaining temperature of the second stage of re-heating is lower than 70° C., Mg—Si clusters promoting the BH responses are not sufficiently generated. Meanwhile, when the attaining temperature of the re-heating is excessively high, since an intermetallic compound phase such as β″ or β′ which is different from the cluster is partially formed, the number density of the cluster is likely to be lower and thus the BH responses are degraded too much. Further, due to β″ or β′, the formability is likely to be degraded.

The cooling to room temperature after the second stage of re-heating treatment may be standing to cool or forced rapid cooling by using the cooling means as in the quenching for efficiency in production. That is, since the clusters whose size is equal or similar to each other, stipulated in the present embodiment, completely come out through the temperature retaining treatment, forced rapid cooling as in a conventional re-heating treatment or controlling the average cooling rate which is performed in a complicated manner over several stages is not necessary.

(Processing with Strain Amount of 0.1% to 5%)

Moreover, in the third embodiment of the present invention, in order to further improve the BH responses, it is preferable that processing with a strain amount of 0.1% to 5% is performed on a sheet from when the solution heat treatment and the quenching treatment are finished to when the re-heating treatment as described below is performed. The means therefor is suitably selected from leveler straightening, skin pass rolling and the like. When the processing with a strain amount of 0.1% to 5% is performed on a sheet from when the solution heat treatment and the quenching treatment are finished to when the re-heating treatment is performed, clusters rich in Mg atoms are more easily generated than clusters rich in Si atoms among the aggregates of atoms satisfying the stipulated conditions and thus the proportion of aggregates of atoms in which the Mg/Si ratio is ½ or greater is likely to be set to 0.70 or greater. Meanwhile, when the strain amount exceeds 5%, which is large, the hem workability is likely to be degraded. Although the mechanism thereof has many unclear points, it is estimated as follows. That is, frozen vacancies are reduced after the solution heat treatment by performing processing on a sheet with a strain amount of 0.1% to 5% after the solution heat treatment and thus the diffusion at room temperature is suppressed. For this reason, it is estimated that the clusters rich in Si generated at room temperature are unlikely to be generated and the proportion of aggregates of atoms in which the Mg/Si ratio is ½ or greater is likely to be set to 0.70 or greater.

(Room Temperature Retention)

Moreover, in order to further improve the BH responses, it is preferable that the room temperature retention time from when the solution heat treatment and the quenching treatment are finished to when the re-heating treatment is started, which includes the processing with a strain amount of 0.1% to 5% described above is set to within 24 hours (hr). When the room temperature retention time is set to be short, the proportion of aggregates of atoms in which the ratio Mg/Si is ½ or greater is likely to be 0.70 or greater. It is preferable that the room temperature retention time becomes shorter. The solution heat treatment, the quenching treatment, and the re-heating treatment may be continuously performed such that there is almost no interval therebetween. The lower limit of the time is not particularly set.

(Re-Heating Treatment)

It is preferable that the attaining temperature of the re-heating treatment is in a temperature range of 80° C. to 160° C. and the retention time is in a range of 3 hrs to 100 hrs. When the attaining temperature of the re-heating is 80° C. or lower or the retention time is shorter than 3 hrs, Mg—Si clusters promoting the BH responses are not sufficiently generated and thus the proportion of clusters in which the ratio Mg/Si is ½ or greater is likely to be less than 0.70. Meanwhile, in a condition where the attaining temperature of the re-heating is higher than 160° C. or the retention time exceeds 100 hrs, since an intermetallic compound phase such as β″ or β′ which is different from the cluster is partially formed, the number density of the cluster is likely to be lower and thus the BH responses are degraded too much. Further, due to β″ or β′, the formability is likely to be degraded.

The cooling to room temperature after the re-heating treatment may be standing to cool or forced rapid cooling by using the cooling means as in the quenching for efficiency in production. That is, since the clusters whose size is equal or similar to each other, stipulated in the present embodiment, completely come out through the temperature retaining treatment, forced rapid cooling as in a conventional re-heating treatment or controlling the average cooling rate which is performed in a complicated manner over several stages is not necessary.

Hereinafter, the present invention will be described more in detail with reference to Examples, but the present invention is not limited to the Examples described below and can be implemented with modifications being added appropriately within the scope adaptable to the purposes described above and below, and any of them is to be included within the technical range of the present invention.

EXAMPLES

Next, Examples of the present invention will be described. First, Examples according to the first embodiment of the present invention will be described. The 6000 series aluminum alloy sheets with different cluster conditions stipulated in the present embodiment were distinctively manufactured by performing the quenching stop at a comparatively high temperature and the retention treatment at the same temperature during the solution heat treatment and the quenching treatment, and the BH responses (bake hardenability) after retention for 7 days and retention for 100 days at room temperature were evaluated. Further, the press formability and the hem workability as the bend workability were also evaluated.

The cluster conditions stipulated in the present embodiment are the average number density of aggregates of atoms, the average radius of the circle equivalent diameters, and the standard deviation of radii of the circle equivalent diameters. The aggregate of atoms is an aggregate of atoms satisfying the conditions that either or both of an Mg atom and an Si atom by a total of 10 pieces or more are contained, in which, when any atom of the Mg atom and the Si atom contained therein is used as a reference, the distance between the atom as the reference and any atom among other atoms adjacent thereto is 0.75 nm or less.

In addition, the 6000 series aluminum alloy sheets having the composition shown in Table 1 were distinctively manufactured variously changing the cooling stop temperature T at a comparatively high temperature in the quenching treatment after the solution heat treatment and the retention time t (h) at the cooling stop temperature as shown in Table 2. Here, in the indication of the content of each element in Table 1, the indication where the numerical value in each element is blank indicates that the content is equal to or less than the detection limit

The specific manufacturing conditions of the aluminum alloy sheets are as described below. The aluminum alloy slab of each composition shown in Table 1 was molten commonly by the DC casting method. Here, commonly to each Example, the average cooling rate during casting was set to 50° C./min from the liquidus temperature to the solidus temperature. Subsequently, the slab was subjected to a soaking treatment at 540° C. for 4 hours commonly to each Example, and then hot rough rolling was started. Further, commonly to each Example, in the finish rolling that followed, hot rolling was performed to a thickness of 3.5 mm to obtain a hot rolled sheet (coil). The aluminum alloy sheet after hot rolling was subjected to rough annealing at 500° C. for 1 minute commonly to each Example and then subjected to cold rolling at a processing rate of 70% without intermediate annealing in the middle of cold rolling pass, whereby a cold rolled sheet (coil) having a thickness of 1.0 mm was obtained commonly to each Example.

Further, a refining treatment (T4) was continuously performed by rewinding and winding the each cold rolled sheet (coil) in a continuous type heat treatment facility commonly to each Example. Specifically, the solution heat treatment and the quenching treatment were performed by heating to the solution heat treatment temperature shown in Table 2 at an average heating rate of 10° C./sec up to 500° C. and then immediately cooling at the average cooling rate shown in Table 2. Here, in each Example in which the quenching (cooling) was stopped at a high temperature and the retention treatment was performed at the same temperature, the quenching and cooling was performed not to room temperature, but the quenching (cooling) was stopped at the quenching stop temperature T shown in Table 2 and then the temperature retention treatment was performed at the temperature for the retention time t (unit h). The temperature retention treatment was performed in a retention furnace retained at each quenching stop temperature in the continuous type heat treatment facility. Moreover, the temperature retention time (measured retention time) t of an actual sheet (coil) was determined to satisfy the formula “1.6×10⁴×exp[−0.096×T]<t<4.3×10⁵×exp[−0.097×T]” in relation to the quenching stop temperature T. The retention times of the lower limits and the upper limits calculated from each quenching stop temperature T by using the formula, and the retention times (measured retention time) of an actual sheet (coil) are shown in Table 2 with the unit h (hour). As a cooling after the temperature retention, in each Example in which the temperature retention was performed, forced rapid cooling at a cooling rate of 100° C./S was performed by using the cooling means as in the quenching.

A test sheet (blank) was cut out from each final product sheet after standing at room temperature for 7 days and 100 days after the refining treatment, and the characteristics of each test sheet were measured and evaluated. Further, the microstructure observation by using the 3DAP was only performed on the samples after 7 days from the refining treatment. These results are shown in Table 3.

(Cluster)

First, the microstructure in the center part in the thickness direction of the test sheet was analyzed by using the above-described 3DAP method, and the average number density (×10²³ pieces/m³), the average radius (nm) of the circle equivalent diameters, and the standard deviation of radii of the circle equivalent diameters of the clusters stipulated in the present embodiment were respectively acquired by using the methods described above. The results thereof are shown in Table 3.

In Table 2, among the conditions of the clusters stipulated in the present embodiment, the containing of either or both of an Mg atom or an Si atom by a total of 10 pieces or more is simply described as “Mg and Si atoms by 10 pieces or more.” Further, when any atom among the Mg atom and Si atom contained therein is used as a reference, the distance between the atom as the reference and any atom among other atoms adjacent thereto being 0.75 nm or less is simply described as “distance of 0.75 nm or less.”

(Bake Hardenability)

After the refining treatment, 0.2% proof stress (As proof stress) and total elongation (As total elongation) were acquired by carrying out a tensile test, as mechanical characteristics of each of the test sheets after standing at room temperature for 7 days or 100 days. Commonly to each test sheet after being subjected to room temperature aging for 7 days and room temperature aging for 100 days, 0.2% proof stress (proof stress after BH treatment) of the test sheet after the artificial aging hardening treatment (after BH) at 185° C. for 20 minutes was acquired by carrying out the tensile test. Then, the BH responses of the each test sheet were evaluated based on a difference (amounts of increase in proof stress) between the 0.2% proof stresses.

With respect to the tensile test, No. 5 specimen (25 mm×50 mmGL×sheet thickness) of JISZ2201 was collected from each sample sheet to perform the tensile test at room temperature. The tensile direction of the specimen was set to a direction orthogonal to the rolling direction. The tensile rate was set to 5 mm/min until the 0.2% proof stress and 20 mm/min after the proof stress. The N number of the measurement of the mechanical characteristics was set to 5, and the average values were calculated. In addition, with respect to the specimen for measurement of proof stress after the above-described BH, 2% pre-strain simulating the press forming of a sheet was applied to the specimen by using a tensile tester and then the BH treatment was performed.

(Hem Workability)

The hem workability was evaluated only for sample sheets after standing at room temperature for 7 days or 100 days after the refining treatment. In the test, by using a strip-shaped specimen having a width of 30 mm, 90° bending of inward bending with 1.0 mm radius by a down flange was performed and, with an inner having a thickness of 1.0 mm inserted therein, the pre-hem working of further bending a bent part inward to approximately 130° in order and the flat hem working of bending by 180° and allowing an end part to be tightly attached to the inner were performed.

The surface state such as occurrence of rough surface, a minute crack or a large crack of the bent part (edge bent part) of the flat hem was visually observed and visually evaluated based on the following criteria.

0: without crack and rough surface, 1: slight rough surface, 2: deep rough surface, 3: minute surface crack, 4: linearly continued surface crack, 5: breakage

As shown in alloy Nos. 0 to 12 of Table 1 and alloy Nos. 0, 1, 7, 13, and 19 to 27 of Table 2, in each Example of Invention, the manufacturing and the refining treatment were performed within the component composition range of the present invention and within the preferable condition range. Therefore, as shown in Table 2, the each Example of Invention satisfies the cluster conditions stipulated in the present embodiment. As a result, in the each Example of Invention, the BH responses are excellent even after the room temperature aging for a long period of time after the refining treatment and even bake-hardened at a low temperature for a short period of time. In addition, as shown in Table 3, even after the room temperature aging for a long period of time after the refining treatment, press formability to an automobile panel is excellent and the hem workability is also excellent because the As proof stress is comparatively low. That is, according to the Examples of Invention, it can be found that an Al—Si—Mg alloy sheet can be provided, which has better BH responses with a proof stress difference of 100 MPa or greater and is capable of exerting press formability or bendability even in a case where the vehicle body bake treatment was carried out after the long-term room temperature aging for 100 days.

In Comparative Examples 2, 8 and 14 of Table 2, Alloy Examples of Invention 1, 2, and 3 of Table 1 are used. However, in these Comparative Examples, the cooling rates of the solution heat treatment are deviated from the preferable condition and excessively low as shown in Table 2. As a result, since the average radius and the standard deviation of the cluster stipulated in the present embodiment are deviated from preferable ranges, and the room temperature aging is high and particularly the As proof stress after retention at room temperature for 100 days is comparatively high, compared to the Examples of Invention which have the same alloy compositions, the press formability or hem workability to an automobile panel is degraded and the BH responses are also degraded.

In Comparative Examples 3 to 6, 9 to 12, and 15 to 18 of Table 2, Alloy Examples of Invention 1, 2, and 3 of Table 1 are used. However, in these Comparative Examples, the temperature retention treatment condition after the quenching stop is deviated from a preferable range as shown in Table 2. As a result, since any of the cluster conditions stipulated in the present embodiment is deviated from the preferable range and particularly the As proof stress after retention at room temperature for 100 days is comparatively high compared to the Examples of Invention which have the same alloy compositions, the press formability or hem workability to an automobile panel or the like is degraded or the BH responses are degraded.

Further, although Comparative Examples 28 to 37 of Table 2 are manufactured in preferred ranges including the temperature retention treatment conditions after the quenching stop, alloy Nos. 13 to 22 of Table 1 are used, in which the contents of Mg, Si, and Sn as indispensable elements are deviated from the ranges of the present invention or the amount of element impurities is excessively large. Therefore, in these Comparative Examples 28 to 37, since particularly the As proof stress after retention at room temperature for 100 days is comparatively high compared to the Examples of Invention, the press formability or hem workability to an automobile panel is degraded or the BH responses are degraded as shown in Table 3. Particularly, in Comparative Example 30 of Table 3 with an excessively small amount of Sn, since the room temperature aging is not suppressed and the As proof stress after retention at room temperature for 100 days is excessively high, the press formability or the hem workability is degraded and the BH responses are less than 100 MPa in terms of an increase amount of proof stress, which is not high. Moreover, in Comparative Example 31 with an excessively large amount of Sn, cracks were generated during hot rolling and thus manufacturing of a sheet itself was not able to be performed.

Comparative Example 28 is an alloy 13 of Table 1 and the amount of Si is excessively small.

Comparative Example 29 is an alloy 14 of Table 1 and the amount of Si is excessively large.

Comparative Example 30 is an alloy 15 of Table 1 and the amount of Sn is excessively small.

Comparative Example 31 is an alloy 16 of Table 1 and the amount of Sn is excessively large.

Comparative Example 32 is an alloy 17 of Table 1 and the amount of Fe is excessively large.

Comparative Example 33 is an alloy 18 of Table 1 and the amount of Mn is excessively large.

Comparative Example 34 is an alloy 19 of Table 1 and the amounts of Cr and Ti are excessively large.

Comparative Example 35 is an alloy 20 of Table 1 and the amount of Cu is excessively large.

Comparative Example 36 is an alloy 21 of Table 1 and the amount of Zn is excessively large.

Comparative Example 37 is an alloy 22 of Table 1 and the amounts of Zr and V are excessively large.

From the results of Examples described above, it can be confirmed that all conditions of the cluster stipulated in the present embodiment are required to be satisfied for the improvement of BH responses after the room temperature aging for a long period of time. Further, in order to obtain such cluster conditions and the BH responses, critical significance or effects of the requirements of the component composition and preferable manufacturing conditions in the present embodiment can also be confirmed.

TABLE 1 Chemical component of aluminum alloy sheet Alloy (mass %, remainder Al) Classification No. Mg Si Sn Fe Mn Cr Zr V Ti Cu Zn Ag Examples of 0 0.6 1.0 0.048 Invention 1 0.6 1.1 0.052 0.2 2 0.4 0.8 0.048 0.2 0.12 0.3 3 0.4 1.2 0.092 0.2 0.21 0.01 4 0.3 1.5 0.050 0.2 0.8 5 0.5 1.3 0.051 0.2 0.7 0.05 6 0.5 0.8 0.207 0.2 0.07 7 0.5 0.9 0.048 0.2 0.22 8 0.6 1.2 0.022 0.2 0.05 0.05 9 1.5 0.5 0.102 0.2 0.1 0.01 10 0.7 1.0 0.050 0.2 0.05 11 0.5 1.2 0.009 0.7 0.6 12 0.5 0.9 0.048 0.2 0.2 0.1 0.1 Comparative 13 1.5 0.2 0.050 0.2 Examples 14 0.4 2.1 0.049 0.2 15 0.6 1.2 0.002 0.2 16 0.6 1.1 0.462 0.2 17 0.4 0.8 0.051 1.3 18 0.6 1.0 0.051 0.2 1.2 0.01 19 0.5 0.8 0.052 0.2 0.4 0.08 20 0.4 0.8 0.050 0.2 1.3 21 0.5 1.0 0.048 0.2 1.2 22 0.5 0.9 0.050 0.2 0.4 0.4 * The columns in which the numerical values for the elements are blank indicate that the values are equal to or less than the detection limit.

TABLE 2 Solution heat treatment and quenching treatment Temperature retention treatment after quenching stop Solution heat Average Cooling stop Average Alloy No. temperature cooling rate temperature T Retention time t cooling rate Classification No. of Table 1 ° C. ° C./s ° C. Measured h Lower limit h Upper limit h ° C./s Example of Invention 0 0 540 100 100 8 1 26 100 Example of Invention 1 1 540 100 100 8 1 26 100 Comparative Example 2 1 540 1 100 5 1 26 100 Comparative Example 3 1 540 100 60 100 50 1276 100 Comparative Example 4 1 540 100 180 0.1 0.0005 0.011 100 Comparative Example 5 1 540 100 100 100 1 26 100 Comparative Example 6 1 540 100 100 0.5 1 26 100 Example of Invention 7 2 540 100 100 8 1 26 100 Comparative Example 8 2 540 1 100 5 1 26 100 Comparative Example 9 2 540 100 60 100 50 1276 100 Comparative Example 10 2 540 100 180 0.1 0.0005 0.011 100 Comparative Example 11 2 540 100 100 100 1 26 100 Comparative Example 12 2 540 100 100 0.5 1 26 100 Example of Invention 13 3 540 100 100 8 1 26 100 Comparative Example 14 3 540 1 100 5 1 26 100 Comparative Example 15 3 540 100 60 100 50 1276 100 Comparative Example 16 3 540 100 180 0.1 0.0005 0.011 100 Comparative Example 17 3 540 100 100 100 1 26 100 Comparative Example 18 3 540 100 100 0.5 1 26 100 Example of Invention 19 4 540 10 80 0.5 7 183 100 Example of Invention 20 5 540 100 130 24 0.06 1 100 Example of Invention 21 6 540 100 90 20 3 70 100 Example of Invention 22 7 540 100 120 1 0.2 4 100 Example of Invention 23 8 540 100 110 2 0.4 10 100 Example of Invention 24 9 540 100 100 4 1 26 100 Example of Invention 25 10 540 100 100 10 1 26 100 Example of Invention 26 11 540 100 100 8 1 26 100 Example of Invention 27 12 540 100 100 8 1 26 100 Comparative Example 28 13 540 100 100 8 1 26 100 Comparative Example 29 14 540 100 100 8 1 26 100 Comparative Example 30 15 540 100 100 8 1 26 100 Comparative Example 31 16 Generation of cracks during hot rolling Comparative Example 32 17 540 100 100 8 1 26 100 Comparative Example 33 18 540 100 100 8 1 26 100 Comparative Example 34 19 540 100 100 8 1 26 100 Comparative Example 35 20 540 100 100 8 1 26 100 Comparative Example 36 21 540 100 100 8 1 26 100 Comparative Example 37 22 540 100 100 8 1 26 100

TABLE 3 Microstructure of aluminum alloy sheet after retention at room temperature for 7 days Stipulated cluster (Mg and Si atoms by Characteristics of aluminum alloy Characteristics of aluminum alloy 10 pieces or more, sheet after retention at room sheet after retention at room distance of 0.75 nm or less) temperature for 7 days temperature for 100 days Average As proof Increase proof Increase Average radius proof stress amount As proof stress amount Hem Alloy density Average standard stress after BH of proof stress after BH of proof work- No. of ×10²³ radius deviation 0.2% 0.2% stress Hem 0.2% 0.2% stress a- Classification No. Table 1 pieces/m³ nm nm MPa MPa MPa workability MPa MPa MPa bility Example of 0 0 6.5 1.25 0.29 94 220 126 1 103 211 108 2 Invention Example of 1 1 7.1 1.26 0.28 97 229 132 1 105 223 118 2 Invention Comparative 2 1 9.4 1.42 0.49 123 199 76 3 130 194 64 3 Example Comparative 3 1 12.9 1.13 0.25 112 183 71 3 129 183 54 2 Example Comparative 4 1 8.4 1.51 0.54 128 219 91 3 143 218 75 4 Example Comparative 5 1 23.7 1.35 0.35 152 264 112 4 158 264 106 4 Example Comparative 6 1 1.8 1.23 0.28 71 177 106 1 92 170 78 2 Example Example of 7 2 3.8 1.25 0.29 77 223 146 1 87 218 131 1 Invention Comparative 8 2 7.2 1.46 0.46 122 193 71 3 130 193 63 3 Example Comparative 9 2 8.4 1.12 0.28 100 176 76 1 115 174 59 2 Example Comparative 10 2 5.8 1.52 0.48 118 209 91 2 127 204 77 3 Example Comparative 11 2 20.4 1.34 0.32 142 261 119 3 148 256 108 4 Example Comparative 12 2 1.6 1.23 0.24 68 173 105 1 84 165 81 1 Example Example of 13 3 9.4 1.30 0.30 103 224 121 2 111 221 110 2 Invention Comparative 14 3 14.3 1.43 0.48 137 209 72 4 142 205 63 4 Example Comparative 15 3 16.6 1.13 0.29 106 177 71 1 119 179 60 2 Example Comparative 16 3 9.3 1.52 0.47 131 224 93 4 138 219 81 4 Example Comparative 17 3 25.5 1.35 0.31 158 266 108 4 162 264 102 4 Example Comparative 18 3 2.1 1.24 0.26 69 167 98 1 83 169 86 1 Example Example of 19 4 3.6 1.28 0.36 106 220 114 2 115 217 102 2 Invention Example of 20 5 15.0 1.23 0.27 108 230 122 2 117 220 103 2 Invention Example of 21 6 9.3 1.25 0.27 99 226 127 2 112 224 112 2 Invention Example of 22 7 10.4 1.22 0.26 92 222 130 2 103 216 113 2 Invention Example of 23 8 6.6 1.41 0.34 94 213 119 2 108 217 109 2 Invention Example of 24 9 9.5 1.32 0.29 97 207 110 2 106 209 103 2 Invention Example of 25 10 9.4 1.33 0.28 101 222 121 2 113 224 111 2 Invention Example of 26 11 13.5 1.28 0.27 108 224 116 2 115 221 106 2 Invention Example of 27 12 11.8 1.30 0.27 102 224 122 2 108 216 108 2 Invention Comparative 28 13 1.3 1.26 0.25 77 129 52 1 82 129 47 1 Example Comparative 29 14 4.5 1.23 0.26 115 211 96 4 118 201 83 4 Example Comparative 30 15 16.8 1.28 0.32 132 235 103 2 139 234 95 3 Example Comparative 31 16 Generation of cracks during hot rolling Example Comparative 32 17 6.1 1.25 0.28 111 218 107 4 118 216 98 4 Example Comparative 33 18 6.4 1.26 0.27 115 218 103 4 122 217 95 4 Example Comparative 34 19 5.8 1.25 0.26 108 219 111 4 117 214 97 4 Example Comparative 35 20 7.5 1.24 0.27 120 224 104 4 126 221 95 4 Example Comparative 36 21 5.1 1.28 0.27 113 220 107 4 121 218 97 4 Example Comparative 37 22 6.6 1.23 0.26 117 226 109 4 129 228 99 4 Example

Next, Examples according to the second embodiment of the present invention will be described. The 6000 series aluminum alloy sheets with different compositions and cluster conditions stipulated in the present embodiment were distinctively manufactured by performing the two-stage re-heating treatment conditions after the completion of the solution heat treatment and the quenching treatment. Further, the microstructure (cluster) and the strength, the BH responses (bake hardenability), and the press formability and the hem workability as the bend workability after the retention at room temperature for 100 days in the each Example were also evaluated.

The cluster conditions are the total amount of Mg atoms and Si atoms present in the aggregates of atoms, the average radius of the circle equivalent diameters, and the average number density. The aggregate of atoms is an aggregate of atoms satisfying the conditions that either or both of an Mg atom and an Si atom by a total of 10 pieces or more are contained, in which, when any atom of the Mg atom and the Si atom contained therein is used as a reference, the distance between the atom as the reference and any atom among other atoms adjacent thereto is 0.75 nm or less.

The specific manufacturing conditions of the aluminum alloy sheets are as described below. The aluminum alloy slab of each composition shown in Table 4 was molten commonly by the DC casting method. Here, commonly to each Example, the average cooling rate during casting was set to 50° C./min from the liquidus temperature to the solidus temperature. In the indication of the content of each element in Table 4 showing the compositions of the 6000 series aluminum alloy sheets of the Examples, the indication where the numerical value in each element is blank indicates that the content is equal to or less than the detection limit and the element is not contained, that is, the value is 0%.

Subsequently, the slab was subjected to a soaking treatment at 540° C. for 4 hours commonly to each Example, and then hot rough rolling was started. Further, commonly to each Example, in the finish rolling that followed, hot rolling was performed to a thickness of 3.5 mm to obtain a hot rolled sheet. The aluminum alloy sheet after hot rolling was subjected to rough annealing at 500° C. for 1 minute commonly to each Example and then subjected to cold rolling at a processing rate of 70% without intermediate annealing in the middle of cold rolling pass, whereby a cold rolled sheet having a thickness of 1.0 mm was obtained commonly to each Example.

Moreover, commonly to the each Example, the cold rolled sheet was subjected to the solution heat treatment in a saltpeter furnace at 560° C., retained for 10 seconds after reaching the target temperature, and subjected to the quenching treatment by water cooling. After the quenching treatment was finished, the first stage of the preliminary aging treatment was performed at 100° C. to 250° C. under the conditions shown in Table 5 and water cooling was carried out until the room temperature. The second stage of the preliminary aging treatment was then performed at 70° C. to 130° C. and cooling was carried out by water cooling until the room temperature. Here, in the present Examples, after the first stage and the second stage of re-heating treatment, cooling was respectively carried out by water cooling, but the same microstructure can be obtained even when the cooling is standing to cool.

A test sheet (blank) was cut out from each sheet after standing at room temperature for 100 days from the refining treatment, and the microstructure and the strength (AS proof stress) of each test sheet were measured. The microstructure observation by using the 3DAP was only performed on the samples after 100 days from the refining treatment. These results are shown in Table 6.

(Cluster)

The microstructures on the sections in the thickness direction of the sheet thickness center parts of the test sheets after the room temperature aging for 100 days were analyzed by using the above-described 3DAP method, and the number density (×10²³ pieces/m³) of the clusters, the average radius (nm) of the circle equivalent diameters and the ratio of the total amount N_(cluster) of the number of all Mg atoms and Si atoms contained in the clusters to N_(total) obtained by summing the number of all measured Mg atoms and Si atoms, stipulated in the present embodiment, were acquired by using the above-described analysis methods.

The results thereof are shown in Table 6. In Table 6, among the conditions of the clusters stipulated in the present embodiment, the containing of either or both of an Mg atom or an Si atom by a total of 10 pieces or more is simply described as “Mg and Si atoms by 10 pieces or more.” Further, when any atom among the Mg atom and Si atom contained therein is used as a reference, the distance between the atom as the reference and any atom among other atoms adjacent thereto being 0.75 nm or less is simply described as “distance of 0.75 nm or less.”

In the measurement according to the 3DAP method, from a test sheet having a thickness of 1 mm, three prisms having dimensions of a length of 30 mm, a width of 1 mm and a thickness of 1 mm were cut out at intervals of 1 mm in the width direction by using a precision cutting device, the prism was processed into a thin shape by performing electrolytic polishing, and then a needle-shaped sample in which the radius of the tip thereof was 50 nm was prepared. Accordingly, the measurement site was in the vicinity of the center portion in the thickness direction. The aluminum alloy sheet sample formed such that the tip had a needle shape was measured according to the 3DAP method by using “LEAP3000” manufactured by Imago Scientific Instruments Corporation. Then, the number density (×10²³ pieces/m³) of the clusters, the average radius (nm) of the circle equivalent diameters, and the ratio of the total amount N_(cluster) of the number of all Mg atoms and Si atoms contained in the clusters to N_(total) obtained by summing the number of all measured Mg atoms and Si atoms, of each of the three prisms, were acquired and then averaged. Therefore, the values in the present Examples are average values of measurement number N=3. The measured volume according to the 3DAP method was approximately 1.0×10⁻²² mm³ to 1.0×10⁻²¹ mm³.

(Bake Hardenability)

0.2% proof stress (As proof stress) and 0.2% proof stress (proof stress after BH) after the artificial aging hardening treatment (after BH) at 185° C. for 20 minutes were acquired as the mechanical characteristics of the each test sheet after the room temperature aging for 100 days, both by carrying out the tensile test. Then, the BH responses of the each test sheet were evaluated based on a difference (amounts of increase in proof stress) between the 0.2% proof stresses.

With respect to the tensile test, No. 5 specimen (25 mm×50 mmGL×sheet thickness) of JISZ2201 was collected from each sample sheet to perform the tensile test at room temperature. The tensile direction of the specimen was set to a direction orthogonal to the rolling direction. The tensile rate was set to 5 mm/min until the 0.2% proof stress and 20 mm/min after the proof stress. The N number of the measurement of the mechanical characteristics was set to 5, and the average values were calculated. In addition, with respect to the specimen for measurement of proof stress after the above-described BH, 2% pre-strain simulating the press forming of a sheet was applied to the specimen by using a tensile tester and then the BH treatment was performed.

(Hem Workability)

The hem workability was evaluated only for sample sheets after standing at room temperature for 7 days or 100 days after the refining treatment. In the test, by using a strip-shaped specimen having a width of 30 mm, 90° bending of inward bending with 1.0 mm radius by a down flange was performed and, with an inner having a thickness of 1.0 mm inserted therein, the pre-hem working of further bending a bent part inward to approximately 130° in order and the flat hem working of bending by 180° and allowing an end part to be tightly attached to the inner were performed.

The surface state such as occurrence of rough surface, a minute crack or a large crack of the bent part (edge bent part) of the flat hem was visually observed and visually evaluated based on the following criteria.

0: without crack and rough surface, 1: slight rough surface, 2: deep rough surface, 3: minute surface crack, 4: linearly continued surface crack, 5: breakage

As shown in alloy Nos. 23 to 32 of Table 4 and alloy Nos. 38, 39, 45, 51, and 57 to 62 of Table 5, in each Example of Invention, the manufacturing and the refining treatment were performed within the component composition range of the present invention and within the preferable condition range. Therefore, as shown in Table 6, the each Example of Invention satisfies the cluster conditions stipulated in the present embodiment. That is, the cluster satisfying the conditions stipulated in the present embodiment satisfies the preferable average number density (3.0×10²⁴ pieces/m³ or greater) and, in the cluster, the ratio (N_(cluster)/N_(total))×100 of N_(cluster) to N_(total) is 1% or greater and 15% or less and the average radius of the circle equivalent diameters is 1.20 urn or greater and 1.50 nm or less.

As a result, in the Examples of Invention, as shown in Table 6, even after the long-term room temperature aging for 100 days or the like, the BH responses are excellent, press formability to an automobile panel is excellent because the As proof stress is comparatively low, and the hem workability is also excellent. That is, according to the Examples of Invention, it can be found that an Al—Si—Mg alloy sheet can be provided, which has better BH responses with a proof stress difference of 100 MPa or greater and is capable of exerting press formability or bendability even in a case where the vehicle body bake treatment was carried out after the long-term room temperature aging for 100 days.

In Comparative Examples 39 to 44, 46 to 50, and 52 to 56 of Table 5, Alloy Examples of Invention 24, 25 and 26 of Table 4 are used. However, in these Comparative Examples, the two-stage re-heating treatment conditions after the completion of the solution heat treatment and the quenching treatment are deviated from the preferable conditions as shown in Table 5.

In Comparative Examples 40, 46 and 52, only one stage that is the second stage of the re-heating treatment is performed.

In Comparative Examples 41, 47 and 53, the re-heating treatment temperature in the first stage thereof is excessively low.

In Comparative Examples 42, 48 and 54, the re-heating treatment temperature in the first stage thereof is excessively high.

In Comparative Examples 43, 49 and 55, the re-heating treatment temperature in the second stage thereof is excessively high.

In Comparative Examples 44, 50 and 56, the re-heating treatment temperature in the second stage thereof is excessively low.

Therefore, in these Comparative Examples, as shown in Table 6, the average radius of the circle equivalent diameters of the aggregates of atoms is less than 1.20 nm or greater than 1.50 nm, or the average ratio of Mg atoms and Si atoms contained in the aggregates of atoms, which is calculated by N_(cluster)/N_(total)×100, is less than 1% or greater than 15%, that is, they are deviated from the stipulation of the present embodiment. As a result, since the As proof stress after retention at room temperature for 100 days is comparatively high, compared to Examples of Invention 39, 45 and 51 which have the same alloy compositions, the press formability or hem workability to an automobile panel is degraded or the BH responses are degraded.

Further, although Comparative Examples 63 to 72 of Table 5 are manufactured in preferred ranges including the refining treatment, alloy Nos. 33 to 42 of Table 4 are used, and the contents of Mg, Si, and Cu as indispensable elements are deviated from the ranges of the present invention or the amount of element impurities is excessively large. As a result, in these Comparative Examples, the BH responses or the hem workability is degraded compared to the Examples of Invention as shown in Table 6. Particularly, in Comparative Example 65 of Table 6 with an excessively small amount of Sn, the number density of the cluster is high and the average ratio (N_(cluster)/N_(total))×100 of N_(cluster) to N_(total) is greater than 15%, which is excessively high. Therefore, since the room temperature aging is not suppressed and the As proof stress after retention at room temperature for 100 days is excessively high, the press formability or the hem workability is degraded and the BH responses are less than 100 MPa in terms of an increase amount of proof stress, which is not high. Moreover, in Comparative Example 66 with an excessively large amount of Sn, cracks were generated during hot rolling and thus manufacturing of a sheet itself was not able to be performed.

Comparative Example 63 is an alloy 33 of Table 4 and the amount of Si is excessively small.

Comparative Example 64 is an alloy 34 of Table 4 and the amount of Si is excessively large.

Comparative Example 65 is an alloy 35 of Table 4 and the amount of Sn is excessively small.

Comparative Example 66 is an alloy 36 of Table 4 and the amount of Sn is excessively large.

Comparative Example 67 is an alloy 37 of Table 4 and the amount of Fe is excessively large.

Comparative Example 68 is an alloy 38 of Table 4 and the amount of Mn is excessively large.

Comparative Example 69 is an alloy 39 of Table 4 and the amount of Cu is excessively large.

Comparative Example 70 is an alloy 40 of Table 4 and the amount of Cr is excessively large.

Comparative Example 71 is an alloy 41 of Table 4 and the amounts of Ti and Zn are excessively large.

Comparative Example 72 is an alloy 42 of Table 4 and the amounts of Zr and V are excessively large.

From the results of Examples described above, it can be confirmed that conditions of the cluster stipulated in the present embodiment are required to be satisfied in order for better BH responses and proof stress after BH to be exerted even in a case where the strength before the bake treatment becomes higher. Further, in order to obtain such cluster conditions and the BH responses, critical significance or effects of the requirements of the component composition and preferable manufacturing conditions in the present embodiment can also be confirmed.

TABLE 4 Chemical component of aluminum alloy sheet Alloy (mass %, remainder Al) Classification No. Mg Si Sn Fe Mn Cu Cr Zr V Ti Zn Ag Examples of 23 0.50 1.00 0.051 Invention 24 0.40 1.25 0.051 0.20 25 0.60 0.90 0.050 0.20 0.7 26 0.50 0.80 0.083 0.20 0.2 27 0.80 0.90 0.049 0.20 0.2 0.15 28 0.70 1.10 0.024 0.20 0.05 0.2 0.2 29 0.40 0.85 0.211 0.20 0.07 0.2 30 0.60 1.00 0.090 0.20 0.4 0.1 31 0.35 1.15 0.050 0.20 0.8 0.6 32 1.00 0.55 0.048 0.70 0.1 0.1 Comparative 33 0.80 0.20 0.049 0.20 Examples 34 0.40 2.10 0.049 0.20 35 0.40 1.00 0.002 0.20 36 0.50 1.00 0.353 0.20 37 0.50 0.90 0.050 1.30 38 0.60 1.10 0.049 0.20 1.1 39 0.60 1.10 0.049 0.20 1.2 40 0.50 0.90 0.050 0.20 0.4 41 0.40 0.90 0.051 0.20 0.08 1.2 42 0.60 1.00 0.052 0.20 0.4 0.4 * The columns in which the numerical values for the elements are blank indicate that the values are equal to or less than the detection limit.

TABLE 5 Between first First stage of and second Second stage of Alloy re-heating treatment stages re-heating treatment No. of Temperature Retention Retention Temperature Retention Classification No. Table 4 ° C. min min ° C. h Example of 38 23 100 2 60 90 8 Invention Example of 39 24 100 2 60 90 8 Invention Comparative 40 24 — — 60 90 8 Example Comparative 41 24 60 2 60 100 8 Example Comparative 42 24 280 2 60 80 8 Example Comparative 43 24 100 2 60 150 8 Example Comparative 44 24 100 2 60 60 8 Example Example of 45 25 100 2 60 90 8 Invention Comparative 46 25 — — 60 90 8 Example Comparative 47 25 60 2 60 100 8 Example Comparative 48 25 280 2 60 80 8 Example Comparative 49 25 100 2 60 180 8 Example Comparative 50 25 100 2 60 60 8 Example Example of 51 26 100 2 60 90 8 Invention Comparative 52 24 — — 15 90 8 Example Comparative 53 26 60 2 15 100 8 Example Comparative 54 26 280 2 15 80 8 Example Comparative 55 26 100 2 15 180 8 Example Comparative 56 26 100 2 15 60 8 Example Example of 57 27 90 5 15 90 8 Invention Example of 58 28 100 2 5 90 8 Invention Example of 59 29 200 2 15 90 5 Invention Example of 60 30 100 2 60 80 3 Invention Example of 61 31 200 2 15 100 3 Invention Example of 62 32 200   0.5 15 90 3 Invention Comparative 63 33 100 2 15 90 8 Example Comparative 64 34 100 2 15 90 8 Example Comparative 65 35 100 2 15 90 8 Example Comparative 66 36 Cracks during hot rolling Example Comparative 67 37 100 2 15 90 8 Example Comparative 68 38 100 2 15 90 8 Example Comparative 69 39 100 2 15 90 8 Example Comparative 70 40 100 2 15 90 8 Example Comparative 71 41 100 2 15 90 8 Example Comparative 72 42 100 2 15 90 8 Example

TABLE 6 Microstructure and characteristics of aluminum alloy sheet after retention at room temperature for 100 days Characteristics of aluminum alloy sheet after Stipulated cluster (Mg and Si atoms by 10 retention at room temperature for 100 days pieces or more, distance of 0.75 nm or less) 0.2% BH responses Alloy Average Number As 0.2% proof stress in terms of No. of radius density N_(cluster)/N_(total) × proof stress after BH increase amount Hem Classification No. Table 4 nm ×10²³ pieces/m³ 100 MPa MPa of proof stress workability Example of Invention 38 23 1.26 9.3 6.2 102 210 108 1 Example of Invention 39 24 1.26 11.6 7.1 107 223 116 1 Comparative Example 40 24 1.18 7.4 4.2 86 175 89 1 Comparative Example 41 24 1.17 6.8 3.9 82 170 88 1 Comparative Example 42 24 1.52 9.0 8.8 138 200 62 3 Comparative Example 43 24 1.48 17.0 15.9 152 205 53 4 Comparative Example 44 24 1.22 1.4 0.8 78 159 81 1 Example of Invention 45 25 1.24 10.5 6.8 109 222 113 2 Comparative Example 46 25 1.18 9.5 5.4 96 182 86 1 Comparative Example 47 25 1.16 5.8 3.3 93 179 86 1 Comparative Example 48 25 1.51 6.2 6.3 146 203 57 3 Comparative Example 49 25 1.44 18.9 16.5 168 224 56 4 Comparative Example 50 25 1.21 1.5 0.9 88 167 79 1 Example of Invention 51 26 1.25 7.5 5.5 104 210 106 1 Comparative Example 52 24 1.18 6.2 4.0 85 168 83 1 Comparative Example 53 26 1.18 6.5 4.4 89 166 77 1 Comparative Example 54 26 1.50 7.1 7.7 144 203 59 4 Comparative Example 55 26 1.46 14.9 15.3 161 223 62 4 Comparative Example 56 26 1.22 1.1 0.8 84 167 83 1 Example of Invention 57 27 1.33 11.9 7.8 122 226 104 2 Example of Invention 58 28 1.34 18.9 11.6 128 239 111 2 Example of Invention 59 29 1.24 4.6 3.6 91 213 122 1 Example of Invention 60 30 1.25 9.4 5.8 106 210 104 1 Example of Invention 61 31 1.25 7.3 4.6 102 209 107 1 Example of Invention 62 32 1.27 4.5 4.4 109 210 101 1 Comparative Example 63 33 1.26 0.6 0.6 85 138 53 1 Comparative Example 64 34 1.23 5.8 2.2 124 201 77 4 Comparative Example 65 35 1.33 19.5 15.5 138 236 98 3 Comparative Example 66 36 Cracks during hot rolling Comparative Example 67 37 1.24 6.5 4.3 125 218 93 4 Comparative Example 68 38 1.26 6.6 3.7 128 223 95 4 Comparative Example 69 39 1.25 6.6 3.8 153 248 95 4 Comparative Example 70 40 1.25 7.3 5.1 122 224 102 4 Comparative Example 71 41 1.27 6.3 4.9 118 210 92 4 Comparative Example 72 42 1.24 5.5 3.3 124 212 88 4

Next, Examples according to the third embodiment of the present invention will be described. The 6000 series aluminum alloy sheets with different compositions and cluster conditions stipulated in the present embodiment were distinctively manufactured by the time from when the solution heat treatment and the quenching treatment were finished to when the re-heating treatment was started, the processing rate of skin pass rolling after the completion of the solution heat treatment and the quenching treatment, and the re-heating treatment conditions. Further, the BH responses (bake hardenability) after the retention at room temperature for 100 days in the each Example were evaluated. Further, the hem workability as the bendability was also evaluated.

In the indication of the content of each element in Table 7 showing the compositions of the 6000 series aluminum alloy sheets of the Examples, the indication where the numerical value in each element is blank shows that the content is equal to or less than the detection limit.

The specific manufacturing conditions of the aluminum alloy sheets are as described below. The aluminum alloy slab of each composition shown in Table 7 was molten commonly by the DC casting method. Here, commonly to each Example, the average cooling rate during casting was set to 50° C./min from the liquidus temperature to the solidus temperature. Subsequently, the slab was subjected to a soaking treatment at 540° C. for 4 hours commonly to each Example, and then hot rough rolling was started. Further, commonly to each Example, in the finish rolling that followed, hot rolling was performed to a thickness of 3.5 mm to obtain a hot rolled sheet. The aluminum alloy sheet after hot rolling was subjected to rough annealing at 500° C. for 1 minute commonly to each Example and then subjected to cold rolling at a processing rate of 70% without intermediate annealing in the middle of cold rolling pass, whereby a cold rolled sheet having a thickness of 1.0 mm was obtained commonly to each Example.

Moreover, commonly to the each Example, the cold rolled sheet were subjected to the solution heat treatment in a saltpeter furnace at 550° C., retained for 10 seconds after the temperature reached the target temperature, and subjected to the quenching treatment by water cooling. After the quenching treatment was finished, the skin pass rolling having a strain amount of 0% to 5% shown in Table 8 was immediately performed in a rolling mill, followed by retaining at room temperature only for the period shown in Table 8 until the re-heating treatment was started. Thereafter, the re-heating treatment was performed under the conditions of the temperature and retention shown in Table 8 by using an atmospheric annealing furnace and then cooled by water after retention for a predetermined time.

A test sheet (blank) was cut out from each final product sheet after standing at room temperature for 100 days from the refining treatment, and the characteristics of each test sheet were measured and evaluated. The microstructure observation by using the 3DAP was only performed on the samples after 100 days from the refining treatment. These results are shown in Table 9.

(Cluster)

First, the microstructures in the center parts in the thickness direction of the test sheets were analyzed by using the above-described 3DAP method, and the average number density (×10²³ pieces/m³) and the average proportion of aggregates of atoms in which the ratio (Mg/Si) of the number of Mg atoms to the number of Si atoms is ½ or greater of the clusters stipulated in the present embodiment were acquired by using the methods described above. The results thereof are shown in Table 9.

Moreover, in Table 9, among the conditions of the clusters stipulated in the present embodiment, the containing of either or both of Mg atoms or Si atoms by a total of 10 pieces or more is simply described as “Mg and Si atoms by 10 pieces or more.” Further, when any atom among the Mg atoms and Si atoms contained therein is used as a reference, the distance between the reference atom and any atom among other atoms adjacent to the reference atom being 0.75 nm or less is simply described as “distance of 0.75 nm or less.”

(Bake Hardenability)

After the refining treatment, 0.2% proof stress (As proof stress) was acquired by carrying out a tensile test, as mechanical characteristic of each of the test sheets after standing at room temperature for 100 days. Commonly to each test sheet after being subjected to room temperature aging for 100 days, 0.2% proof stress (proof stress after BH) of the test sheet after the artificial aging hardening treatment (after BH) at 185° C. for 20 minutes was acquired by carrying out the tensile test. Then, the BH responses of the each test sheet were evaluated based on a difference (amounts of increase in proof stress) between the 0.2% proof stresses.

With respect to the tensile test, No. 5 specimen (25 mm×50 mmGL×sheet thickness) of JISZ2201 was collected from each sample sheet to perform the tensile test at room temperature. The tensile direction of the specimen was set to a direction orthogonal to the rolling direction. The tensile rate was set to 5 mm/min until the 0.2% proof stress and 20 mm/min after the proof stress. The N number of the measurement of the mechanical characteristics was set to 5, and the average values were calculated. In addition, with respect to the specimen for measurement of proof stress after the above-described BH, 2% pre-strain simulating the press forming of a sheet was applied to the specimen by using a tensile tester and then the BH treatment was performed.

(Hem Workability)

The hem workability was evaluated for sample sheets after standing for 100 days after the refining treatment. In the test, by using a strip-shaped specimen having a width of 30 mm, 90° bending of inward bending with 1.0 mm radius by a down flange was performed and, with an inner having a thickness of 1.0 mm inserted therein, the pre-hem working of further bending a bent part inward to approximately 130° in order and the flat hem working of bending by 180° and allowing an end part to be tightly attached to the inner were performed.

The surface state such as occurrence of rough surface, a minute crack or a large crack of the bent part (edge bent part) of the flat hem was visually observed and visually evaluated based on the following criteria.

0: without crack and rough surface, 1: slight rough surface, 2: deep rough surface, 3: minute surface crack, 4: linearly continued surface crack, 5: breakage

As shown in alloy Nos. 43 to 52 of Table 7 and alloy Nos. 73, 74, 80, 86, and 92 to 97 of Table 8, in each Example of Invention, the manufacturing and the refining treatment were performed within the component composition range of the present invention and within the preferable condition range. Therefore, as shown in Table 8, the each Example of Invention satisfies the cluster conditions stipulated in the present embodiment. That is, the cluster satisfying the conditions stipulated in the present embodiment satisfies the preferable average number density (3.0×10²⁴ pieces/m³ or greater) and, in the cluster, the average proportion of aggregates of atoms in which the ratio (Mg/Si) of the number of Mg atoms to the number of Si atoms is ½ or greater is 0.70 or greater.

As a result, in the Examples of Invention, as shown in Table 9, even after the long-term room temperature aging from the refining treatment, the BH responses are excellent, press formability to an automobile panel is excellent because the As proof stress is comparatively low, and the hem workability is also excellent. That is, according to the Examples of Invention, it can be found that an Al—Si—Mg alloy sheet can be provided, which has better BH responses with a proof stress difference of 100 MPa or greater and is capable of exerting press formability or bendability even in a case where the vehicle body bake treatment was carried out after the long-term room temperature aging for 100 days.

In Comparative Examples 75, 81 and 87 of Table 8, Alloy Examples of Invention 44, 45 and 46 of Table 9 are used. However, in these Comparative Examples, as shown in Table 8, the time from when the solution heat treatment and the quenching treatment are finished to when the re-heating treatment is started is extremely long. As a result, as shown in Table 9, although the average number density (×10²³ pieces/m³) of the clusters stipulated in the present embodiment satisfies the stipulation, the average proportion of aggregates of atoms in which the ratio (Mg/Si) of the number of Mg atoms to the number of Si atoms is ½ or greater is excessively small, and the room temperature aging is high and particularly the BH responses after the retention at room temperature for 100 days are degraded compared to Examples of Invention 74, 80 and 86 which have the same alloy compositions.

In Comparative Examples 76, 82 and 88 of Table 8, Alloy Examples of Invention 44, 45 and 46 of Table 9 are used. These Comparative Examples are manufactured under the preferable manufacturing conditions except for the skin pass rolling after the solution heat treatment and the quenching treatment as shown in Table 8. For this reason, the average number density (×10²³ pieces/m³) of the clusters stipulated in the present embodiment satisfies the stipulation. However, since the skin pass rolling (work) is not performed, as shown in Table 9, among the aggregates of atoms satisfying these conditions, the average proportion of aggregates of atoms, in which the ratio (Mg/Si) of the number of Mg atoms to the number of Si atoms is ½ or greater is excessively small. Therefore, the room temperature aging is high and particularly the BH responses after the retention at room temperature for 100 days are degraded compared to Examples of Invention 74, 80 and 86 which have the same alloy compositions.

In Comparative Examples 77 to 79, 83 to 85, and 89 to 91 of Table 8, Alloy Examples of Invention 44, 45 and 46 of Table 7 are used. However, in these Comparative Examples, the re-heating treatment conditions are deviated from the preferred ranges as shown in Table 8. Accordingly, the average number density of the aggregates of atoms or the average proportion of the aggregates of atoms, in which the ratio (Mg/Si) of the number of Mg atoms to the number of Si atoms is ½ or greater, is excessively small. Therefore, the BH responses after the retention at room temperature for 100 days or the hem workability is degraded compared to Examples of Invention 74, 80 and 86 which have the same alloy compositions.

Further, although Comparative Examples 98 to 107 of Table 9 are manufactured in preferred ranges including the refining treatment, alloy Nos. 53 to 62 of Table 7 are used, and the contents of Mg, Si, and Sn as indispensable elements are deviated from the ranges of the present invention or the amount of element impurities is excessively large. For this reason, in these Comparative Examples, particularly the BH responses or the hem workability after the retention at room temperature for 100 days is degraded compared to the Examples of Invention as shown in Table 9. Particularly, in Comparative Example 100 of Table 9 with an excessively small amount of Sn, since the room temperature aging is not suppressed and the As proof stress after retention at room temperature for 100 days is excessively high, the press formability or the hem workability is degraded and the BH responses are less than 100 MPa in terms of an increase amount of proof stress, which is not high. Moreover, in Comparative Example 101 with an excessively large amount of Sn, cracks were generated during hot rolling and thus manufacturing of a sheet itself was not able to be performed.

Comparative Example 98 is an alloy 53 of Table 7 and the amount of Si is excessively small.

Comparative Example 99 is an alloy 54 of Table 7 and the amount of Si is excessively large.

Comparative Example 100 is an alloy 55 of Table 7 and the amount of Sn is excessively small.

Comparative Example 101 is an alloy 56 of Table 7 and the amount of Sn is excessively large.

Comparative Example 102 is an alloy 57 of Table 7 and the amount of Fe is excessively large.

Comparative Example 103 is an alloy 58 of Table 7 and the amount of Mn is excessively large.

Comparative Example 104 is an alloy 59 of Table 7 and the amount of Cr is excessively large.

Comparative Example 105 is an alloy 60 of Table 7 and the amount of Cu is excessively large.

Comparative Example 106 is an alloy 61 of Table 7 and the amounts of Ti and Zn are excessively large.

Comparative Example 107 is an alloy 62 of Table 7 and the amounts of Zr and V are excessively large.

From the results of Examples described above, it can be confirmed that all conditions of the cluster stipulated in the present embodiment are required to be satisfied for the improvement of BH responses after the room temperature aging. Further, in order to obtain such cluster conditions and the BH responses, critical significance or effects of the requirements of the component composition and preferable manufacturing conditions in the present embodiment can also be confirmed.

TABLE 7 Chemical component of aluminum alloy sheet Alloy (mass %, remainder Al) Classification No. Mg Si Sn Fe Mn Cr Zr V Ti Cu Zn Ag Examples of 43 0.55 1.05 0.050 Invention 44 0.50 1.10 0.049 0.20 45 0.35 1.10 0.049 0.20 0.12 0.2 46 0.60 0.90 0.094 0.20 0.01 0.21 0.01 47 0.30 1.00 0.051 0.20 0.8 48 0.50 1.25 0.048 0.20 0.7 0.2 49 0.50 0.90 0.206 0.20 0.07 0.22 0.1 50 0.40 0.85 0.009 0.20 0.05 0.15 51 0.60 1.15 0.049 0.70 0.05 0.05 0.6 52 0.80 0.55 0.024 0.20 0.1 0.01 Comparative 53 0.80 0.20 0.049 0.20 Examples 54 0.40 2.10 0.051 0.20 55 0.40 1.15 0.003 56 0.40 1.10 0.455 57 0.50 0.90 0.050 1.30 58 0.50 1.10 0.049 0.20 1.1 59 0.45 0.80 0.048 0.20 0.4 60 0.50 0.80 0.049 0.20 1.3 0.01 61 0.40 0.90 0.049 0.20 0.08 1.2 62 0.60 1.00 0.049 0.20 0.4 0.4 * The columns in which the numerical values for the elements are blank indicate that the values are equal to or less than the detection limit.

TABLE 8 Time from completion of solution heat treatment and quenching treatment Skin pass rolling Re-heating treatment Alloy No. to start of re-heating treatment processing rate Temperature Retention No. of Table 7 hr % ° C. h Example of Invention 73 43 1 3 100 8 Example of Invention 74 44 1 3 100 8 Comparative Example 75 44 48 3 100 8 Comparative Example 76 44 1 — 100 8 Comparative Example 77 44 1 3 60 12 Comparative Example 78 44 1 3 180 3 Comparative Example 79 44 1 3 100 1 Example of Invention 80 45 1 3 100 8 Comparative Example 81 45 48 3 100 8 Comparative Example 82 45 1 — 100 8 Comparative Example 83 45 1 3 60 12 Comparative Example 84 45 1 3 180 3 Comparative Example 85 45 1 3 100 1 Example of Invention 86 46 1 3 100 8 Comparative Example 87 46 48 3 100 8 Comparative Example 88 46 1 — 100 8 Comparative Example 89 46 1 3 60 5 Comparative Example 90 46 1 3 180 3 Comparative Example 91 46 1 3 100 1 Example of Invention 92 47 1 3 100 8 Example of Invention 93 48 16 3 100 8 Example of Invention 94 49 1 1 80 16 Example of Invention 95 50 1 5 100 8 Example of Invention 96 51 1 3 90 12 Example of Invention 97 52 1 3 120 3 Comparative Example 98 53 1 3 100 8 Comparative Example 99 54 1 3 100 8 Comparative Example 100 55 1 3 100 8 Comparative Example 101 56 Generation of cracks during hot rolling Comparative Example 102 57 1 3 100 8 Comparative Example 103 58 1 3 100 8 Comparative Example 104 59 1 3 100 8 Comparative Example 105 60 1 3 100 8 Comparative Example 106 61 1 3 100 8 Comparative Example 107 62 1 3 100 8

TABLE 9 Microstructure of aluminum alloy sheet after retention at room temperature for 100 days Stipulated cluster (Mg and Si atoms by 10 pieces or Characteristics of aluminum alloy sheet after retention at more, distance of 0.75 nm or less) room temperature for 100 days Proportion in which As 0.2% 0.2% proof stress Increase amount Alloy No. Average density Mg/Si ratio is proof stress after BH of proof stress Hem Classification No. of Table 7 ×10²³ pieces/m³ 1/2 or greater MPa MPa MPa workability Example of Invention 73 43 7 0.84 114 226 112 2 Example of Invention 74 44 11 0.83 118 233 115 2 Comparative Example 75 44 15 0.65 121 191 70 2 Comparative Example 76 44 6 0.67 106 196 90 1 Comparative Example 77 44 8 0.62 80 148 68 1 Comparative Example 78 44 2 0.87 146 213 67 4 Comparative Example 79 44 7 0.66 111 192 81 2 Example of Invention 80 45 9 0.81 103 215 112 2 Comparative Example 81 45 15 0.64 120 186 66 2 Comparative Example 82 45 7 0.65 100 193 93 2 Comparative Example 83 45 6 0.60 77 157 80 1 Comparative Example 84 45 2 0.85 148 208 60 4 Comparative Example 85 45 8 0.62 114 199 85 2 Example of Invention 86 46 9 0.79 116 224 108 2 Comparative Example 87 46 14 0.64 123 195 72 2 Comparative Example 88 46 8 0.61 106 202 96 2 Comparative Example 89 46 11 0.58 117 179 62 2 Comparative Example 90 46 2 0.80 151 217 66 4 Comparative Example 91 46 8 0.61 115 202 87 2 Example of Invention 92 47 6 0.79 103 209 106 1 Example of Invention 93 48 15 0.72 128 245 117 2 Example of Invention 94 49 9 0.85 106 217 111 2 Example of Invention 95 50 8 0.84 89 208 119 1 Example of Invention 96 51 16 0.81 124 232 108 2 Example of Invention 97 52 6 0.93 96 202 106 2 Comparative Example 98 53 1 0.88 83 129 46 1 Comparative Example 99 54 15 0.68 120 211 91 4 Comparative Example 100 55 14 0.74 132 230 98 3 Comparative Example 101 56 Generation of cracks during hot rolling Comparative Example 102 57 8 0.82 125 223 98 4 Comparative Example 103 58 12 0.78 126 222 96 4 Comparative Example 104 59 4 0.86 128 224 96 4 Comparative Example 105 60 5 0.92 147 251 104 4 Comparative Example 106 61 5 0.78 132 223 91 4 Comparative Example 107 62 9 0.81 128 217 89 4

The present invention has been described in detail with reference to specific embodiments, but it is clear for a person with an ordinary skill in the art that various alterations and modifications can be added without departing from the spirit and scope of the present invention.

Further, the present application is based on Japanese Patent Application (Japanese Patent Application No. 2013-185197) filed on Sep. 6, 2013, Japanese Patent Application (Japanese Patent Application No. 2013-185198) filed on Sep. 6, 2013 and Japanese Patent Application (Japanese Patent Application No. 2013-185199) filed on Sep. 6, 2013, and the contents of which are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to provide a 6000 series aluminum alloy sheet having a combination of the BH responses under the conditions of a low temperature for a short period of time after room temperature aging for a long period of time and also the formability after room temperature aging for a long period of time. Further, it is possible to provide a 6000 series aluminum alloy sheet which is capable of exerting better BH responses even in a case of room temperature aging in which the strength before the baking becomes higher. As a result, applications of the 6000 series aluminum alloy sheet can be expanded as members and components of transportation machines such as an automobile, a marine vessel and a vehicle, household electric appliance, buildings, and structures, and particularly as members of transportation machines such as an automobile. For example, in addition to panel materials for an automobile, the sheet is preferred in a case of being used for pillars such as a center pillar, arms such as a side arm, and reinforcing materials such as bumper reinforcement and door beam which are skeleton members or structural members of an automobile, and also being used as thin plates for skeleton members or structural members of other than an automobile. 

1. An aluminum alloy sheet, which is an Al—Mg—Si alloy sheet comprising, in mass %: Mg: 0.2% to 2.0%; Si: 0.3% to 2.0%; Sn: 0.005% to 0.3%; Al; and inevitable impurities, comprising an aggregate of atoms when measured by a three-dimensional atom probe field ion microscope, wherein: either or both of a Mg atom and a Si atom are contained in the aggregate of atoms by a total of 10 pieces or more and, when any atom of the Mg atom and the Si atom contained therein is used as a reference, a distance between the atom as the reference and any atom among other atoms adjacent thereto is 0.75 nm or less; the aggregate of atoms satisfying these conditions is contained in the aluminum alloy sheet at an average number density of 2.5×10²³ pieces/m³ or greater and 20.0×10²³ pieces/m³ or less; an average radius of a circle equivalent diameter of the aggregate of atoms satisfying these conditions is 1.15 nm or greater and 1.45 nm or less; and a standard deviation of the radius of the circle equivalent diameter is 0.45 nm or less.
 2. The aluminum alloy sheet according to claim 1, further comprising at least one of, in mass % Mn: more than 0% and 1.0% or less, Cu: more than 0% and 1.0% or less, Fe: more than 0% and 1.0% or less, Cr: more than 0% and 0.3% or less, Zr: more than 0% and 0.3% or less, V: more than 0% and 0.3% or less, Ti: more than 0% and 0.1% or less, Zn: more than 0% and 1.0% or less, and Ag: more than 0% and 0.2% or less.
 3. An aluminum alloy sheet excellent in bake hardenability, which is an Al—Mg—Si alloy sheet comprising, in mass %: Mg: 0.2% to 2.0%; Si: 0.3% to 2.0%; Sn: 0.005% to 0.3%; Al; and inevitable impurities, wherein a ratio (N_(cluster)/N_(total))×100 of N_(cluster) to N_(total) is 1% or greater and 15% or less; N_(total) represents a total number of all Mg atoms and Si atoms measured by a three-dimensional atom probe field ion microscope; N_(cluster) represents a total number of all Mg atoms and Si atoms contained in all aggregates of atoms satisfying conditions wherein an aggregate of atoms measured by the three-dimensional atom probe field ion microscope contains either or both of a Mg atom and a Si atom by a total of 10 pieces or more and, when any atom of the Mg atom and the Si atom is used as a reference, a distance between the atom as the reference and any atom among other atoms adjacent thereto is 0.75 nm or less; and an average radius of a circle equivalent diameter of the aggregate of atoms is 1.20 nm or greater and 1.50 nm or less.
 4. The aluminum alloy sheet according to claim 3, further comprising at least one of, in mass % Mn: more than 0% and 1.0% or less, Cu: more than 0% and 1.0% or less, Fe: more than 0% and 1.0% or less, Cr: more than 0% and 0.3% or less, Zr: more than 0% and 0.3% or less, V: more than 0% and 0.3% or less, Ti: more than 0% and 0.1% or less, Zn: more than 0% and 1.0% or less, and Ag: more than 0% and 0.2% or less.
 5. An aluminum alloy sheet excellent in bake hardenability, which is an Al—Mg—Si alloy sheet comprising, in mass %: Mg: 0.2% to 2.0%; Si: 0.3% to 2.0%; Sn: 0.005% to 0.3%; Al; and inevitable impurities, comprising an aggregate of atoms when measured by a three-dimensional atom probe field ion microscope, wherein: either or both of a Mg atom and a Si atom are contained in the aggregate of atoms by a total of 10 pieces or more and, when any atom of the Mg atom and the Si atom contained therein is used as a reference, a distance between the atom as the reference and any atom among other atoms adjacent thereto is 0.75 nm or less; and the aggregate of atoms satisfying these conditions has an average number density of 3.0×10²³ pieces/m³ or greater and 25.0×10²³ pieces/m³ or less and, among the aggregates of atoms satisfying these conditions, an average proportion of an aggregate of atoms wherein a ratio (Mg/Si) of a number of Mg atoms to a number of Si atoms is ½ or greater is 0.70 or greater.
 6. The aluminum alloy sheet according to claim 5, further comprising at least one of, in mass % Mn: more than 0% and 1.0% or less, Cu: more than 0% and 1.0% or less, Fe: more than 0% and 1.0% or less, Cr: more than 0% and 0.3% or less, Zr: more than 0% and 0.3% or less, V: more than 0% and 0.3% or less, Ti: more than 0% and 0.1% or less, Zn: more than 0% and 1.0% or less, and Ag: more than 0% and 0.2% or less. 